mm ^^trnm^ \%\ CORNELL UNIVERSITY LIBRARY FROM Cornell University Library QL 821.02 The principal «o™s of the skel^^^^^^^ " 1Q?4 002 913 089 ^^ THE PEINCIPAL FOEMS SKELETON AND THE TEETH; AS THE BASIS ]?0B » A SYSTEM OF NATURAL HISTORY AND COMPARATIYE ANATOMY. t!»^ PEOFESSOE OWEN. rmof a single many-jointed ray, retaining the archetypal character. Fig. 7, No. 53. In other fishes, the number of rays progressively increase, untU, in those called "rays" par excellence, they exceed a hundred in. number, and are of great length, forming the chief and most conspicuous parts of the fish. The more common condition of the appendage in question is that exhibited in the species figured. Cut 9. So developed, it is called in Ichthyology the " pectoral fin :" otherwise and variously modified in higher animals, the same part becomes a fore-leg, a wing, an arm, and hand. Some of the special names, originally applied to the parts of the scapular appendage in man, are retained and appUed to like parts in the pectoral fin of the fish. Of the two flat bones : connecting the fin with the coracoid, the upper one is the " jJ^^^l^Sili ' ^^ lower one the " radius," No. 55 ; the row of short bones joined with these are the " carpalS)" No. 66; the longer and more slender many-jointed rays answer to the parts called "metacarpals" and " phalanges" in the hxunan hand. In the salmon there is a bone ■nsweiling to the arm-bone or humerus, which is articulated to the middle of the back 176 PABIETAIi SEGMENT, OB VEETBBBA. part Of the coracoid by a transversely elongated extremity. It is also expanded at the diatal end, where it articulates by cartilage with the ulna and radius. The ulna is a semicircular plate of bone perforated in the centre, and, besides its articulation with the humerus, the radius, and the ulnar carpals and metacarpal ray, italso directly joins the broad coracoid. The radius, after expanding to unite with the humerus, the ulna, and the radial carpals, sends a long and broad process downwards and inwards, which is united by Ugament with its fellow and with the lower termination of the coracoid. A basis of adequate extent and firmness is thus insured for the support of the pectoral fins. - The carpal bones of these fins are four in number, progressively increasing in length from the ulnar to the radial side of the wrist. The metacarpo-phalangial rays are thirteen in number ; the uppermost or ulnar one being the strongest, and articulating directly with the ulna. Proceeding to the next segment, in advance, in the cod-fish's skull, we find that the bone which articulated with the centrum of the occipital segment is continued forward beneath a great proportion of the skull. In quadrupeds, however, the corresponding part of the base of the skull is occupied by two bones ; and if the single long bone in the fish be sawn across at the part where the natural suture exists in the beast, we have then little difficulty in disarticiilating and bringing away with it a series of bones similar in number and arrangement to those of the occipital segment. In the skeletons of most animals the centrums of two or more segments become, in certain parts of the body, confluent, or they may be connate ; they form, in fact, one bone, like that, e. g., which human anatomists call " sacrum." By the term " con- fluent" is meant the cohesion or blending together of two bones which were ori- ginally separate ; by " connate," that the ossification of the common fibrous or cartilaginous bases of two bones proceeds from one point or centre, and so con- verts such bases into one bone . this is the case, e. g., in the radius and ulna of the frog, and in its tibia and fibula. In both instances they are to the eye a single bone; but the mind, transcending the senses, recognises such single bone as being essentially two. In like manner it recognises the "occipital bone" of man as essentially four bones ; but these have become " confluent," and were not " connate." The centrums of the two middle segments of the fish's skull are connate, and the little violence above recommended is requisite to detach the penultimate segment of the skull. When detached, the bones of it are seen to be so arranged as to form a neural and a hfflmal arch. In the neural arch the centrum, neurapophyses, diapophyses, and neural spine are distinct ; moreover, the neural spine in the cod, and many other fishes, is bifid, or split at the median line.* The centrum is called " basisphenoid," No. 5 ; the neurapophysis, " alisphenoid," No. 6 ; the neural spine, " parietal," No. 7 ; and the diapophysis, " mastoid," No. 8 . The alisphenoids protect the sides of the optic lobes, and the rest of the penultimate segment of the brain ; the mastoids project outwards and backwards as strong transverse processes, and give attachment to the piers of the great inverted haemal arch. Before noticing the structure of this, I may remark that, in the recent cod-fish, the case, partly gristly, partly bony, which contains the organ of hearing, is wedged in between the last and penultimate neural arches of the skull. The extent to which the ear-case is ossified varies in difierent fishes, but the bone is always developed in the outer waU of the case. In the cod-fish it is unusually large, and is called "petrosal," No. 16; it forms no part of the segmented neuroskeleton. In the • " Archetype Verl. Skel.," p. 11, Fig. 2. mONTAIi SEGMENT, OB, VERTEBEA. 17" organ -which it contributes to inclose, there is a body as hard as shell, liie half a split almond : it is the " ptosteal," No. 16 , or proper ear-b6ne. % The haemal arch consists of a pleurapophysis and a hicmapophysis on each side, and a haemal spine ; but all these elements ' are subdivided, the pleurapophysis into two parts, the upper one called " epitympanic," 28 , a (common to this and the nest arch in advance) ; the lower one " stylohyal," No. 38 . The haemapophysis is a broader, slightly arched bone ; the upper division is called "epihyal," No. 39 ; the lower division, " coratohyal," No, 40 . The haemal spine is subdivided into four stumpy bones, called collectively " basihya l," No. 41 ; and which, in most fishes, support a bone directed forwards, entering the substance of the tongue, called " glossohyal," No. 42 ; and another bone directed backwards, called " urohyal," No. 43 , 4he ceratohyal part of the haemapophysis supports, in the cod, seven long and slender bent bones, called " branchiostegal rays," 44, The number of these rays differs greatly in different fishes : the protopterus has but one ray, the blenny has two rays, the carp three rays, — a very common number is seven ; but the slops has thirty bran- chiostegal rays. They are of great length in the angler-fish [lophius), in which they serve to support a menjbrane, developed to form a large receptacle on each side of the head of this singular fish ; into these receptacles, the small fishes are transferred, which the angler attracts within reach of its mouth, by the movable rod, line, and bait attached to the top of its enormous head. In ordinary fishes, the branchiostegal rays support a membrane which helps to close the gill-slit, and by its movements contri- butes to the direction of the branchial currents. It is an appendage, or rudimental Umb, answering to the pectoral fin diverging from the haemal arch, in the adjoining occipital segment. The penultimate segment of the skull above described is called the " parietal ver- tebra ;" and the haemal areh is called the " hyoidean arch," in reference to its support- ing and subserving the movements of the tongue. The next segment, or the second of the skull, counting backwards, can be detached from the foremost segment vrithout dividing any bone. It is then seen to consist, Uke the third and fourth segments, of two arches and a common centre ; but the constituent bones have been subject to more extreme modifications. The centrum, called " pre- sphenoid," No. 9, is produced far forwards, slightly expanding ; the neurapophyses, called " orbitosphenoids," No. 10 , are small semioval plates, protecting the sides of the cerebrum ; the neural spine, or key-bone of the arch, caUed " frontal," No. 11 , is enor- mously expanded, but in the cod and most fishes is single ; the diapophyses, called " post-frontals," No. 12, project outwards from the hinder angles of the frontal, and give attachment to the piers of the inverted haemal arch. The first bone of this arch is common in fishes to it and to that of the last described vertebra, being the bone called " epitympanic," No. 28 (Fig, 9) ; this modification is called for by the ne"essity of consejitaneous movements of the two inverted arches, in connection with the degluti- tion and course of the streams of water required for the branchial respiration. The haemal arch of the present segment — enormously developed — is plainly divided primarily on each side iato a pleurapOBhysis and haemiapophysis ; for these elements are joined together by a movable articulation, whilst the hones into which they are subdivided are suturaUy interlocked together. The pleurapophysis is so subdivided into four pieces ; the upper one, articulating with the post-frontal and mastoid — ^the diapophyses of the two middle segments of the skull — is called " epitympani c," No. 28, a ; the hind- most of tie two middle pieces is the " roeeotympanicT^ No. 28, b ; the foremost of the 1?!J ilASAi SEGMENT, OK VeKTEEKA. two miefdle pieces is the' " pretyflipanic ," No. 28, c ; the lower piece is tfio hypotym- panio. No. 28, d ; this jrtesenta a joint-Burface, convex in one way, coHcare in the other, called a " ginglymoici condyle," for the haemapophysis, or lower divisioB of the arch. In most air-hreathing yertebrates^-the serpent^ Cut 16,- e. ^.^-the pleitrapophysia resumes its normal simplicity; and is a single bone, 28, which is called the "tympanic ;" in the eel-tribe it is in two pieces. The greater subdivision, in more actively breathing fishes, of the tympanic peddcle^ gives it additional elasticity, and by their overlapping, interlocking junction, greater resistance against fracture ; and these qualities seem to have been requrred in consequence of the presence of a complex and largely -ileveloped diverging appendage, which forms the framework of the principal flap or door, called " operculum," that opens and closes the branchial fissure on each side. The appendage in question consists of four bones ; the olte articulated to the tympanic pedicle is csLUed " preopercular," No. 34 ; the other three are, counting downwards, the " opercular," No. 35; the "subopercular," No. 36; the " interopercular," No. 37. The hssmapophysis is subdh"ded into two, three, or more pieces, in different fishes, suturaUy interlocked togetner ; the most common division is into two subequal parts, one presenting the concavo-convex joint to the plenrapophysis, and called " articular," No. 29 ; the other^ bifiu-cated behind to receive the point of 29, and joining its fellow at the opposite end; to complete the heemal arch : it is very singularly modified by supporting, and having more or less firmly attached to it, a number of the hard bodies, called " teeth," and hence it has been termed the " dentary," No. 33. In the cod there is a small separate bone, below the joint of the attticular, forming an angle there, and called the " angular piece," No. 31. In consequence of this extreme modification, in relation to the offices of seizing and Setiag upon the food, the pair of haBmapophyses of the present segment of the skull have deceived the name of " lower jaw," or " mandible" {mandibula). The entire segment k called the " frontal vertebra." The first segment, forming the anterior extremity, of the neuroskelcton, like most pierfphe*al partsy is that which has undergone the most extreme modifications. The obvious arramgement, nevertheless, of its constituent bones, when viewed from behind, stfte* its detachment from the second segment, affords one of the most conclusive proofs of the principle of afflierence to common type which governs aU the segments of the neuroskelcton, -Whatever offices they may be modified to fulfil. The neural arch plainlv exists, but is now ieduced to its essential elements — viz., the centrunif the neur- apophyses, and the aeural spine. The centrum is expanded anteriorly, where it usually supports some teetl* on its under surface in fishes ;■ it is called the " vomer " No. 13. The neuJapophyses aire notehed (in the cod), or perforated (in the sword-fish) by the crura or prolongations of the brain, which expand into its anterior divisions' called " olfactory lobes ;" the special name of such neurapophysis is " prefrontal " No 14. The neural spine is usually single,- sometimes cleft along the middle • it is the i' nasal," Nol 15. The hffimal arch is drawn forwards, so that its a(pex, as well as its piers, are ioined to the centrum (vomer) and usually also to the neural spine (nasal), closing up ante- riorly the neurd canal. The pleurapophyses are sijmple, short, sending backwards an expanded plate : they are called " palatines," No. 20. The haimapophyses are simple and their essential part, intervening between the pkurayopb-yais and hiemal spine "' short and thick; but they send a long process backwards, Thiff clement is colled "iSMftillary," No. 41. The heemal spine,, cleft at the nuddle liaie, sends one process GBNEKAL AND SPEOIAL NAMES 6f BONES. 179 upwards of varying length in different fishes, tiii A second djbwnwarda and backwards • and its imder surface is beset with teeth ift most fishes : it is called " premaxiUary," No. 22. Each pleurapophysis supports a " diverging appendage," consisting commonly of two bones : the outer one, which fixes the present haemal arch to the succeeding one is called " pterygoid," NO'. 24 j Ifce iimer one is the " entopterygoid," No. 23. The entire segment is called the " nasEii vertebra." The haemal arch and its appendage form what is termed the upper jaw (maxilla) ; the palatine and pterygoids forming the roof of the mouth, the maxillary and premaxiUary the proper upper jaw. On reviewing the arrangement of the bones of the foregoing segments, one cannot but be struck by the strength of the arches which protect and encompass the brain, and by the bearity and efficiency of that arrangement which provides such an arch for each primary divi- sion of the brain ; and a sentiment of admiration naturally arises on examining the' firm interlocking of the extended sntural surfaces, and especially of those uniting the proper elements of the arch with the buttresses wedged in between the piers and key- stone, and to which buttresses (diapophyses) the larger haemal arches are suspended. In addition to the parts of the neuroskeleton, the bones of the head include the ossified part of the ear-capsule, "petrosal," 16, already mentioned; an ossified part of the eye-capsule, commonly in two pieces, " sclerotals," No. 17 ; and sta ossified part of the capsule of the organ of smell, " turbinal," No. 19. Another assemblage of splanohnoskeletal bones support the gills, and are in the form of slender bony hoops, called " branchial arches." They are articulated to and supported by the hyoidean arch. Amongst the bones of the muco-dermal system, may be noticed those that circumscribe the lower part of the orbit, of which the anterior is pretty constant in the vertebrate series, and is called " laorymal," marked 20 in Cut 9. In fishes they are called " suborbitals," and are occasionally present in great numbers, as, e. ^., in the tunny. A similar series of bones sotaetimes overarches the temporal fossae, and are called " supertemporals." At the outset of the study of Osteology it is essenti'al to know well the numerous bones in the head of af fiah, and to fix in the memory their arrangement and names. The latter, as we have seen, are of two kinds, as regards the bones of the neuro- skeleton ; the one kind is " geh'eral," indicative of the relation of the skuU-bon^s to the typical segment, and' which names they bear in common with the same elements in the segments of the trunk ; the other kind is " special," and bestowed on- account of the particular development and shape of such elements, as they are' modified in the head for particular functions. I would advise any one earnestly desirous of comprehending this beautiful department of Comparative Anatomy to obta^in a' prepared and partially disarticulated skull of a cod-fish from Mr. Flower,* in which every bone bears the initials of its "general" name, and the numerals indicative of its "special" name. A great proportion of the bones in the head of a; fish exist in a very Similar state of connection and arrangement in the heads of ofher vertebrata', up to and including man himself. No method could be less coiiducive to a true and philosophical comprehension 6f the vertebrate skeleton than the beginning its study in man— the most modified of all vertebrate forms, and that which recedes furthest from the common pattern. Through an inevitable ignorance of that pattern, the bones in anthropotomy are indicated only by special names more or less relating to the particular forms these bones happen to bear m man ; such nameB, When applied to the tallying bones in lower animals, losing that • Ante,'^. 173.- 180 CLASSIFICATION OP THE BONES OF THE HEAD. significance, and becoming grtitrary sag^s. Owing to the frequent modification hy con- fluence of the humai; bones, collections of them, so united, haye received a single name, as, e. g., " occipital," " temporal," &c. ; whilst their constituents, which are usually distinct vertebral elements, have received no names, or are defined as processes, c, g., " condyloid process of ^he occipital boi;e," " styloid, process of the temporal bone,'' " petrpus portio;i of the temporal bone," &c. The eiasoification, moreover, of the bones of the head in Human Anatomy, viz., into those of the cranium and those of the face, ia artificial or special, and consequently defective. Many bo^es -virhich essentially belong to the s^ull are wholly omitted in such classificatiou. In regar4 to t^e archetype of the vertebrate skeletoi^ fishes, which were the first forms of vertebrate life introduced into this planet, deviate the least there&om ; and according to the foregoing analysis of the boqes of tbe head, H foPows that such bones are primarily divisible into those of— The Keuroskeleton ; The Splanchnoskeletou ; The Dermoskeleton. The ueuroskeletal bones are arranged in four segments, called-^ The Occipital segment ; ■Hie Parietal segment ; The Frontal segment ; The Nasal segment. Each segment consists of a " neural" and a " hsemal" arch. The neural arches are— ■ N I. EpencephaUc arch (boiies Nos. 1, 2, 3, 4) ; N n. Mesencephalic arch (5, 6, 7, 8) ; N m. Prosencephalio arch (9, 10, 11, 12) ; K IV. Ehinencephalic arch (13, 14, 15). The haemal arches are — H I. Scapular arch (50-52) ; H n. Hyoidean arch (38-43) ; H nt Mandibular arch (28-32) ; H IV. MaxiUary arch (20-22). The diverging appendages of the hsemal arches are — 1. The Pectoral (54-57) ; 2. The Branchiostegal (44) ; 3. The Opercular (34-37) ; 4. The Pterygoid (23-24). The bones or parts of the splanchno-skeleton which are intercalated with or attached to the arches of the true vertebral segmsnts, are — The Petrosal (16) or ear-capsule, with the otolites, 16" • The Sclerotal (17) or eye-capsule ; The Turbinal (19) or nose-capsule ; The Branchial arches ; The '^eeth. kODIFICATIONS OF THE JaVs OF FISHES. ISl The bones of the dermoskeleton are — The Supratemporals ; The Superorbitala ; The Suborbitals ; The Labials. Such appears to be the natural classification of the palrts whith constitute thd oomplez skuU of osseous fishes. The term "cranium" might weU be applied to the four neural arches collectively, but would exclude some bones called " cranial," and include some called " facial," in Human Anatomy. In a side view of the naturally-connected bones of the head of a fish, such as is shown in the figure of the skeleton of the sea-perch. Cut 9, the upper part of the head is formed by the neural spines called superoocipital, 3j frontal, 11, and nasal, 15 : produced at the hinder half into the median ridge. The right lateral ridge is formed by the parietal, 7, and paroccipital, 4 ; the external ridge by the post- firontal, 12, and the mastoid, 8. The anterior termination of the series of centrums may be partly seen through the widely-open orbits at 9 and iS, indicating the pre- sphenoid and vomer respectively. The most conspicuous parts of the upper jaw are the prcmazillary, 22, and the maxiUaryj 21, the latter being edentulous, as in most fishes : the salmon and trout are examples where No; 21 bears teethi The shape and slight attachment of those bones relate to the necessity of a movable mouth that can be pro- truded and retracted, in a class of animals that derive no aid in the prehension of their food from their Hmbs, which are reduced to fins. The upper bent back part of the premaxUlary is called its " nasal branch," and is of unusual length in fishes With protractile snouts, as, e. g., the dories [Zeus); certain wrasses {Corieus), and especially the slybream [Sparaa insidiator of PaUas). In this fish the nasal branch of the pre- maxillary plays in a groove on the upper surface of the skull, and reaches as far back as the occiput, when the mouth is shut and retracted. The descending branch of the pre- maxillary is attached by a ligament to the maxUlary, and, as this is similarly attached to the mandible, both are protruded, when the long nasal branch of the premaxillary is drawn forwards out of its epicranial groove. This action is aided by the hypotympanic, which is of great length, and has a movable articulation at both ends ; the lower end joining the mandible is pulled forward, simultaneously with the protrusion of the premaxillary, and co-operates therewith in the sudden projection of the mouth, by which the sly-beam seizes, or shoots with a suddenly-propelled drop of ^iter, the small agile aquatic insects that constitute its prey. An opposite extreme of modification of the maxillary and premaxillary bones, where unusual fixity and strength are needed, is that presented by the " sword-fishes," in which the premaxillaries constitute, by an unusual prolongation and density of tissue, the sword-shaped weapon characteristic of the genera Xiphias and Istiophorus. In Cut 9 the divisions 28 a, e, and rf, of the tympanic pedicle, and the two chief divisions, 29 and 33, of the mandible, are shown, together with the four bones of the opercular appendage ; the preopercular, 34, being serrated and spined, as in most perches. Of the hyoidfian arch may be seen the glossohyal, 42, the ceratohyal, 40, with its branchiostegal rays, 44, and the urohyal, 43. Of the scapular arch, the scapula, 51, and the coracoid, 52, this supports not only the bones of the " pectoral fin," P, viz., ulna, radius, with the small carpal bones intervening between them and the metacarpo- 182 MODIFICATIONS OF THE CATTDAL VBRTEBRjE. Fig. 10. m phalanges, 67, but also the lower elements of the pelvic arch, 63, and their diverging appendage, 69, called the " ventral fin," V. In the segments of the trunk the ha3mapophyses, save ia the first vertebra, 58, and the pelvic vertebra, 63, are not ossified ; but they are represented by apo- neurotic fascia continued downwards from the ossified elements of the segments; these elements consist of the centrum, the neurapophyses, and neural spines, jJie pleurapophyses, and the parapophyses. In most fishes the neural spines are connate with the neurapo- physes, and these become confluent with the centrums in most of the segments : the neurapophyses are per- forated directly by the spinal nerves in many fishes, aa at MM (Fig. 8) ; they usually develop anterior zygapo- physes, Z (Fig. 9). The centrums are biconcave in all fishes, save the lepidosteus, in which they are convex in front, and concave behind. The pleurapophyses, pi (Fig. 9), form what are called " false ribs," or free or " floating ribs," in Anthropotomy : they articulate with the ceijtrums in the anterior trunk-vertebrse, and then with the parapophyses, p, which are usually confluentwith the centrum. The parapophyses elongate, bend down, and unite together at or near to the end of the abdomen, and so form the contracted haemal canal, for the caudal vessels, in the long and muscular tail of the fish. The trunk-vertebrae of a fish are divisible into those which have free pleurapophyses, caEed " abdominal vertebras," and those without, and which terminate below by narrow haemal arches and long spines, called " caudal vertebrae." These haemal arches are formed by different parts in difierent fishes : commonly by the bent-down and terminally confluent parapophyses (Fig. 10), I, p, cod; sometimes, as in the tunny {ib.) Ill, by parapophyses, p, lengthened out by pleurapophyses, pi ; sometimes, as in lepidosteus (ii.), II, by pleurapophypses, pi; but never, as in air- breathing vertebrates {ii.), V, by ossified hsmapo- physes. A, As. Thesi' elements, in the first vertebra of the trunk of a fish, are, indeed, ossified, and form the long and slender bone called " clavicle, " 58 (Fig. 9) usually attached to the inner side of the scapular arch. The hfflmapophyses of, probably, the last abdominal vertebra, called " ischia," No. 63, are detached from the rest of their segment, and are either loosely sus- pended in the flesh, beneath or near it, as in the fishes called "abdominal;" or they are advanced, much elon- gated, and attached to the scapular arch, as in the (Fig. 9) ; or they are more advanced, shortened, and fishes called "thoracic" STaTJGTURE ASTD FORMULA OV THE FHST-KAYS. 183 similarly attached, aa in the fishes called "jugular;" or they are -srholly wanting, as in the fishes called "apodal." The fins called "ventral," Y, supported by the pelyic haemapophyses, indicate by their position the orders of fishes called " abdo- minal," "thoracic," and "jugular," by Liimseus. The only proper fins in pairs are the " pectoral," P, answering to the fore-limbs of quadrupeds, and the "ventral," V, answering to the hind-limbs. The rest of the fins are single and median iu position, and are due to folds of the skin, in which certain dermal bones are developed for their support. These bones are of two kinds : one, dagger-shaped, are plunged, so to speak, up to the hilt, in the flesh between the neural spines, and between the haemal spines ; those along the upper surface of the fish are called " interneural spines," in, Cut 9 ; those on the under surface are the " inter- hsemal sprues," i/i. The iutemeural spines support the " dermoneural spines, dn, forming the rays of the dorsal fin or fins, Dl, D2, and the upper rays of the caudal fin. The interhgemal spiues support the dermohaemal spines, dh, which form the rays of the anal fin. A, and the lower rays of the caudal fin, d/i, C. Both dermoneural and dermohiemal spines may present two structures ; they may be simple, unjointed, firm, bony spines; orthey may be flexible, jointed, and branched rays. Those fishes which have one or more of the hard spines, at the beginning of the pectoral, ventral, dorsal, and anal fins are called " acanthopterygian," or spiny-finned fishes (Gr. aocmtlios, spine ; pterux, fin) ; those in which the vertical fins are supported by soft spines are called " malaoopterygian," or soft-finned fishes (Gr. malakos, soft ; stadpterux). Ichthy- ologists avail themselves of the number and kind of rays in the fins to characterize the spe- cies of fishes, and adopt an abbreviated formula and symbols to express these characters. In regard to the sea-perch (Fig. 9), the fin-formula would be as follows : — D 7, 1 -I- 12 : P 12 : V 1 + 5 : A 3 4- 8 : C 18, which signifies that D, the dorsal fin, has, in its first division, 7 rays, all spinous : in its second division, 1 spinous -j- (plus) 12 rays that are soft. P, the pectoral fin, has 12 rays, all soft. V, the ventral fin, has 1 spinous + 5 soft rays. A, the anal fin, has , 3 spinous -\- 8 soft rays. C, the caudal fin, has 18 rays. When the piscine modification of the vertebrate skeleton is contemplated in relation to the life and movements of a fish in its native element, every departure from the archetype is seen to be in direct relation to the habits and well-being of the species. The large head has been compared to the embryonic disproportion of that part iu higher vertebrates ; but the head of a fish should be of the size and shape best fitted to overcome the resistance of water, and to facilitate rapid progression through ■ that element : the head must, therefore, grow with the growth of the body. Accordingly, the large skull-bones always show the radiating bony filaments in their clear circum- ference, which is the seat of growth ; and hence the number of overlapping squamous ' sutures which least oppose the progressive extension of the bones. The cranial cavity expands with the expansion of the skuU, but the brain undergoes no corresponding increase ; it lies at the bottom of its capacious chamber, which is principally occupied by a ^ose cellular tissue, situated, like the " arachnoid" membrane in man, between the brain-tunics, called " pia mater" and " dura mater," and having its cells filled by a light, oily fluid ; thus the head is rendered specifically lighter than if growth only, and not the modelling absorption also, had gone on. The loose connection of the haemal arches and their parts, including most of what are called " bones of the face," seems like the retention of a condition observable in the paitiaUy-developed skuU of the embryos of higher animals ; but this condition is subservient to the peculiar and exteUr 184 ADAPTATION OF *HE FISh's SKUtL TO AQUATIC ttFE. sivo movements of the jaws, and of the bony supports of the breathing machinery. Not any of the limbs of fishes are prehensile ; the mouth may be propelled by them to the food, but the act of taking it must be performed by the jaws ; these can, accordingly, be not only opened and shut, but can be protruded and retracted. The division of the long tympanic pedicle into several partlyoverlapping pieces adds to its strength, and ' by a slight elastic yielding diminishes the liability to fracture. The tongue, to judge by its structure, seems to serve little as an organ of taste, but the arch sustaining it has much to effect in the way of swallowing . for this action relates not merely to food j the mechanical part of breathing is a modified, habitilal, and frequent act of deglutition. The hyoid arch is the chief support of the branchial arches and giUs ; and the branchiostega) membranes, stretched out upon the diverging rays of the hyoid arch, regulate the coui'se and exit of the respiratory currents. By the retraction of the hyoid arch the opercular doors are forced open, and the branchial cavity is widened, whilst all entry from behind is prevented by the brauohi- OBtegal flaps, which close the external gill-openings. The water, therefore, enters by the gaping mouth, and rushes through the sieve-like interspaces of the branchial arches into the branchial cavity ; the mouth then shuts, the opercular doors close upon the branchial and hyoid arches, which again swing forwards ; and the branchiostegal mem- branes being withdrawn, the currents rush out at the gill-openings. Thus the mecha.' nieal functions of the haemal arches of the thorax of the higher air-breathing classes are transferred to the haemal arches and appendages of the skuU in fishes. The persistent gills and gill-arches in fishes have been compared with the same parts ■which are transitory in frogs, and with some traces of branchial or^nizatiou in the embryos of higher vertebrates : and fishes have been called, in the language of the transmutation-of-speoies hypothesis, " arrested gigantic tadpoles." It wiH be found, however, that so far from there having been any stoppage of development, the branchial arches have been adapted to the exigencies of the fish by advancing to a grade of structure which they never reach in the frog. This ia shown by their firm ossification, and their numerous elastic joints ; the sieve-Mke valves developed from the side next the mouth have been pre-arranged, -with the utmost complexity and nicety of adjust- ment, to prevent the entry of any particles of food, or other irritating matters, into the interspaces of the tender, vascular, and sensitive gills. It is interesting, also, further to observe, that the last pair of these arches, which, when the embryo-fish is as yet edentulous, usually support gills, are reduced, -when the supply of yolk-food is exhausted, and the jaws get their prehensile organs, to the capacity of the gullet become thickened, in order to support teeth for tearing in pieces, mincing, or crushing the food, and are converted into an accessory pair of jaws, and this pair the most important of the two, as it would seem ; for the carp-tribe — e. g., tench, Imrbel roach— which have no teeth on their proper jaws, have teeth on the pharyngeal jaws. In no other vertebrate animals, save the osseous fishes, is the mouth provided with maxil- lary instruments at both the fore and hind apertures ; and in no other part of the piscine straoture is the direct divergence from any conceivable progressive scale of ascending organisms, culminating in man, so plainly marked as in this. The general form of the fish is admirably adapted to the element in which it lives and moves. The viscera are packed in a moderate compass, in a cavity broueht for wards close to the head. The absence of any neck gives the advantage of a more exten- sive and resisting attachment of the head to the trunk, and a greater proportion of th trunk is left free for the allocation of the muscular masses which move the tail ' T« ABAMATION OP THB FISH'S SKtTLTi TO AQUATIC LIFE. 185 the " caudal" division of the vertebral coliinm, the parapophyses cease to extend out- wards ; they bend downwards, unite and elongate in that direction, proportionally with the elongation of the spines above, whilst dermal and intercalated spines shoot forth from the middle line above and below, giving the vertically extended, compressed form to the hinder half of the body, by the alternating lateral strokes of which the fish is propelled forwards it the diagonal between the direction of those forces. The advan- tage of the biconcave form of vertebra, with intervening elastic capsules of gelatinous fluid, in producing a combination of the resilient with the muscular power, is as obvious as it is beautiful to contemplate. The fixation and coalescence of any of the vertebra in this locomotive part of the fishes' body, analogous to the part called "sacrum" and "pelvis" in land c[uadrupeds, would be a great hindrance to the alternate and vigorous inflections of that part, by which mainly the fish swims. A " sacrum" is a consolidation of part of the vertebral axis of the body, for the transference of more or less of the weight of that body upon limbs organized for its support on dry land ; such a modification would have been not merely useless, but a hindrance to a fish. The pectoral fins are the prototypes of the fore-limbs of the higher vertebrates. With their terminal segment, or " hand," alone projecting fireely irom. the trunk, and swathed in a common sheath of skin, they present an interesting analogy to the embryonal buds of the answerable members in man. But what would have been the result if both arm and fore-arm had extended freely from the side of the fish, and dangled as a long many-jointed appendage in the water ! This " higher development," as it is termed, in relation to the prehensile or cursorial limb of the denizen of dry land, would have been a defect in the structure of a creature destined to cleave the liquid element. In the fish, therefore, the fore-limb is left as short as was compatible with its required functions : the broad, many-fingered hand alone projects, biit can be applied prone and flat, by flexion of the wrist, to the side of the trunk ; or it may be extended with its flat surfaces turned forwards and backwards, so as to check and arrest, more or less suddenly, the progress of the fish ; its breadth can also be diminished by closing up or stretching out the digital rays. In the act of flexion, the pectoral fin slightly rotates, and gives an oblique stroke to the water. The requisite breadth of the modified hand is gained by the addition of ten, twenty, or it may be a hundred fingers over and above the number to which they are restricted in the fore foot or hand of the higher classes of vertebrata. The pike maintains a stationary position in a stream by vibrations of the pectoral fins : the nature of the bottom of the fish's habitat is ascertained by a tactile application of the same fins. In the hard^aced gur- nards certain rays of the pectorals are liberated from the web, and have a special endowment of nerves, in order to act as feelers. In the siluroid fishes, the pectorals wield a formidable weapon of offence. A tropical species of perch {Anabas) uses a smaller analogous pectoral spine for climbing up the mangrove stems inquest of insects. Certain lophioid fishes that live on sand banks left dry at low water, are enabled to hop after the retreating tide by a special prolongation of the carpal joint of the pectoral fin, which projects in these " frog fishes," as they have been termed, like the limb of a land quadruped, and presents two distinct segments clear of the trunk. The sharks, whose form of body and strength of tail enable them to swim near the surface of the ocean, are further adapted for this sphere of activity, and compensated for the absence of an air-bladder, by the large proportional size of their pectoral fins, which take a greater share in their active and varied evolutions than in ordinary fishes ; more eaBecially in producing that half turn or roll of the body required to bring the 186 ACTION OP THE PINS IN PISHES, InOTith, which is on the under part of the head, in contact with their prey. The maxi- mum of development of the many-fingered hands is attained in the rays, and in those fishes — e.g., JSxocatus and Sactylopterus — called " flying fishes," in consequence of the iectorals being long enough, and their webs broad enough to sustain them in the air, ix their long " flying leaps" out of the water. With regard to the ventral fins — ^the rudiments of hind-limbs — these combine merely with the pectorals in raising the fish, and in preventing, as outriggers, the rolling of the body during progression. In the long-bodied and small-headed abdominal fishes the ventrals are situated near the vent, where they best subserve the office of accessory balancers ; in the large-headed thoracic and jugular fishes they are transferred forwards, to aid the pectorals in supporting and raising the head. If the pectoral and ventral fins' in one of these fishes be cut off, the head sinlu to the bottom ; if the right pectoral fiij only be cut off', the fish leans to that side ; if the ventral fin on the same side be cut away, then it loses its equilibrium entirely ; if the dorsal and anal fins be cut off, the fish reels to the right and left ; when the caudal fill is out off, the fish loses the power of progressive motion ; when the fish dies, and the fins cease to play, the belly turns upwards. Paley thus sums up the actions of the fins of fishes: — " The pectoral, and more par- ticularly the ventral, fins serve to raise and depress the fish ; when the fish desires to have a retrograde motion, a stroke forward with the pectoral fin effectually produces it • if the fish desire to turn either way, a single blow with the tail the opposite way sends it round at once ; if the tail strike both ways, the motion produced by the double lash is progressive, and enables the fish to dart forwards with an astonishing velocitji. The result is not oniy in some cases the most rapid, but in all cases the most gentle, pliant, easy animal motion with which we are acquainted." " In their mechanical use, the anal fin may be reckoned the keel ; the ventral fins, the outriggers ; the pectoral fins, the oars ;" and we may now add " the caudal fin, the screw-propeller." And if there be such simUitud , between those parts of a boat and a fish, " observe," adds Paley, " that it is not the vesemblanoe of imitation, but the likeness which arises from applying similar mechanical means to the same purposes." * Fxincipal Forms of the Skeleton in the Class Reptilia The transi- tion from fishes to reptiles is easy, and the signs thereof very manifest in the skeleton. In the thomback and allied fishes the skull articulates with the trunk by two condyles and the part answering to the basioccipital is a depressed plate. The Batraehia or lowest order of reptiles— including the siren, proteus, frog, toad— have a similar double articu- lation of '^he skuU with the trunk, the two condyles being developed from the two exocoipitals. Hamapophyses are not present as bones in the abdominal part of the trunk of Batraehia, but they are so developed in the taU. This structure with the detachment of the scapular arch from the occiput, and the absence of dermoneural and dermohsemal spines, serves to distinguish the most fish-like batrachian from the pro- toptcrus and lepidosiren, which are the most reptile-like of fishes. In commencing the study of the skeletons of reptiles in the most fish-like of the class, we find a much less complex condition of the osseous framework of the body than in the bony fi»t3s ; this will be immediately manifest by a comparison of the skeleton of the menopome (which may be seen in the Museum, Eoyal College of Surgeons No 683), as an example of the perennibranchiate batraehia, with the skeleton of the trout (No. 45) or of the haddock (No. 176, in the same Museum), • • " Nat. Theology," 8vo, 1805, p. 257. BATRACHIAN ILLtlSTRATIONS OF THE NATUEB OF LIMBS. 187 Fig. 11. The difference tends greatly to elucidate the true nature of the complexities of the fish's skeleton, since it chiefly consists in the simplification of that of the batrachian, by the non-development of the parts of the dermal skeleton -which charaotej-ize that of the fish. The suborbital, superorbital, and supratemporal soa}eTboij.e3 are removed, together with the opercular bones, from the head ; aT).d the interiieural aijd dermoneural spines, with the interhsemal and dermohaemal spines, are removed from the trunk. The endoskeleton is also reduced to a very simple cojjdition ; the advance characteristic of the higher class being appreciable only by a comparison of it with the skeleton of the most batraohoid of fishes — e, g., the protopterus (No. 380). We then perceive that the bodies of the vertebrae, in the true batrachian, aro distinctly ossified, though Jireserving, ia the peren- nibranchiate species, a deep, conical, jelly-filled cavity both beforfe and be- hind (Cut 11), C ; they have also coalesced with the neural arches, as these have with their spines, which are, however, scarcely pro- minent, except in the tail. The transverse processes are developednotonly from the centrum tut from the base of the neural archj and are formed by both pMapophyses and diapophyses ; and they coexist with distinct hsemapophyses in the tail {ib.), H. With these, likewise, coexist cartilaginous pleur- apophyses ((i.), pi, in the second, third, and fourth caudal vertebrae ; short ossified plfiirapophyses being developed from the ends of the diapophyses iu the first caudal to the vertebra dentata inclusive. By this instructive condition of the skeleton of the menopome, we perceive at once that the hsemapophyses {ib.), H, are neither transverse processes, nor ribs bent down or displaced, but are elements of vertebrae, as distinct as the neurapophyses above. The neural arches are now articulated together by well-developed zygapophyses with syno- vial articulations, which are absent in the protopterus, as iu most fishes. In the protopterus, as iu the squatina and some other cartilaginous fishes, the neural arch of the atlas rests upon a backward production of the basioccipital ; ui the batrachians it is confluent with its own proper centrum, which developes two articular surfaces for the two occipital condyles. The haemal arch of the occipital segment, which is attached to its proper vertebra in the protopterus (Fig. 32), A, 61, 52, as in osseous fishes, is detached and displaced backwards in the batrachians (Fig. 33), 51, 52. In the com- pletion of the haemal arch of the sacral vertebra iu the menopome, by the enlarge- ment of its 'transverse process (Fig. 11), D, and by its pleurapophysis {ib.),pl, extended to join a haemapophysis {ib.), H, below, we have the key to the essential nature of the pelvis in all air-breathing animals. The progressive development of the appendages of the scapular and pelvic arches, which are to become the four Umbs of air-breathing vertebrates, should be traced from their condition in the protopterus. Here (Fig. 32) they are reduced to a single ray, which is soft and many-jointed. In the Amphimna 9ACEA;L TEETliBEji AND CONTfOUOUS TEBTZBKiB — UENOPOUE. 188 MEtAJNtOEfHOSES OP THE FBOG'S SKILETONi didaetyla (Fig. 33) the ray is ossifled : its firat joint {ib.), 53, is long, its second («*.), 5i, 55, is bifid, and a cartilage at the end of this supports two short terminal rays. This is the pattern of the subdiviaioi of the appendage both of the scapular and pelvic arches, in all the higher vertebrates : hence, in consequence of the vast modifications of the several segments, the necessity for their special names. In the fore-Umb the first segment (Kg. 33), 63, is the " armj" aid its bdne, the " humerus," No 53 ; the second segment is the fore-arm— its two bones are the "radius,"' No. 65, and "ulna," No. 54; the third segment ia the " haild"— its rays are the "fingers;" and its bones are subdivided into "carpals," No. 66, "metacarpals," and "phdlanges," No. 57. In ► the hind-limb (Fig. 34) the Jim segment ia the "thigh," and its bone, the "femur," No. 65 ; the second segment is the "leg," and its two bones are the "tibia" No. 66, and "fibula," No. 67; the third segment is the "foot" — its rays are the "toes;" ita bones are subdivided into "tarsals," "metatarsals," and "phalanges." In the siren the pelvic arch and limbs are not developed ; but they coexist with the scapular arch and Umbs in all other batracHa. In the proteus the last segment of the fore' limb divides into three rays, that of the hind-limb into two rays ; in other words, it has three fingers and two toes. The menobranchus has four fingers and four toes. The axolotl has four fingers and five toes. The menopome has five fingers and five toes. The ultimate subdivisions of the radiated or diverging appendages of the scapular and pelvic arches do not exceed five in any existing air-breathing aMmal, and their further complexity is due to the specialization of each digit, so as to combine iii associated action, instead of their indefinite multipHcation; ■vtrhich causes the seeming complexity of the same appendages in fishes. In all the fish-lite batrachia, called, from a retention of ra.oi6 dr l^ss df the branchial apparatus, " perennibranchia," the limbs are short, and the rays Of the terminal segments of each Umb are, more or less, united by a web : the body is long, and the tail long and compressed. But a great ascent in the scale of life is made in the batrftchian order : all the species when hatched have the fish-like form, and giUs for breafhiilg water ; most of them exist for some time, under this form, ill water ; and these undergo so strange a modification of form and structure before arriving at maturity, that it has been called a " metamorphosis.*' They change their aquatic for a terrestrial life ; they breathe air instead of water ; and from being onmivoroua become ciimivorous. The tsidpoles of our common toad and frog afTord ready and abundant instdndes for traciiig these stages. The following is an outline of the main phdhomena of the change observable in regard to the osseous system : — In the development of the skeleton of the coitimon frog, a fibrous and cartilaginous framework is originally laid down cdnformably with the aqilatic habits and life of the larva. A large cartilaginous cranuim with four haemal arches, and one of these supporting the framework of the branchial apparatus, — a short series of fibro-cartilaginous verte- brae, minus the haemal arches, iri the trunk, and a series of flbrOus septa diverging from the fibrous capsule of the notochdrd, and defining tad giving attachment to the mus- cular segments along the tail,— constitute the skeletoil of the newly hatched tadpole. As it grows, ossification begins ; but only in those part of the skeleton which are to be retained in the future frog. Thus, the dentrunls and neurapophyses of the head and trunk are ossified, but not those of tho tall. In the trunk, ossification of the vertebral body proceeds centripetally by layers, successively diminishing in extent, and conical interspaces are left, consisting of the changed fibrous capsule of the notochord with the inclosed gelatinous cells, their liquefied contents forming the balls of fluid, between the METAMOKPHOSl^S 05 THE FK,OG's SKULL. 189 hioonoave vertebrae, as in fishes. But ossiflcation proceeds to fill up the hinder cavity at the ceatnun, and to project into the front cavity of the succeeding vertebra, -with -vrhich it is finally connected by a synovial ball-and-socket joint. Thus, the firmer intervertebral articulations are established, which adapt the vertebral column to the support of a body which is to be suspended upon limbs, and transported by them along the surface of the dry ground. Whilst this change \b proceeding, the tail is undergoing rapid absorption, the retained fibro-oartilaginous condition of its vertebrae rendering them more ready for removal. In the last fused rudiments of the caudal vertebrse, ossification extends oon-i tinuously, and the peculiar style (Fig. 12), c, at the end of the vertebral series in the frog and other tail-less batrachians, is thus established. In the conversion of the biconcave into cup-and-ball vertehrsa in batraohian larvae, pssification commonly, but not always, pro^ ceeds to obliterate the hinder cavity. In ^^ ^^W' ^^^ the land salamanders, ho'^^^Ter, it extends from the front cavity j so that in the adult vertebrae the baU is anterior, and the cup posT tenor, as in certain salamaudroid fishes — e. g., lepidosteus. In those batrachians that retain more or less of the branchial apparatus, with the outward form and natatory tail adapted to aquatic life, the vertebrae of the tail are ossified hie those of the trunk, but the bicour cave structure and intgrvening gelatinous joints are retained throughout life. The chief changes which take place in the conversion of the oartilagiflous skull of the larva to the ossided one of the imago, or pei-foct frog, are seen in the shape and relative position of the haemal arches and their appendages^-s. e., of the maxillary, mandibnlar, hyoid, and scapular arches. The maxillary arch expands in breadth, the mouth widens, and the horny mandibles are shed. As the mouth advances forwards, the tympanic pedicles are elongated, and are placed more obliquely ; their proximal end retrograding from the post-frontal to the mastoid region of the skuU, and their dis- tal end inclining forwards with the attached lowerjaw,Nos. 29,33, on which the denticles now begin to be developed. Forthe still more extraordinary changes of the hyoid arch. No. 41, and its branchial appendages. No. 46, the student is referred to Dugfe's "Eecher- ches sur I'Osteologie des Batraciens," 4to, 1835; and to the writer's "Archetype of the Vertebrate Skeleton," pp. 70, 71. The scapular arch, which was close to the occiput, whilst protecting and supporting; SKELETON OF TBB FBOo {liana esculenta). 190 SKELETON OF THE FEOG. tho branchdal heart — it3 primary function — begins, as the rudiments of the fore- limbs bud out, to recede backwards, liie the mandibular and branchial arches, but to a greater extent, the attachment to the occipital segment being wholly lost. The scapular and coraooid portions of the arch become first ossified ; the suprascapular plate remains long cartilaginous, and always partly so ; the sternum is developed in pro- portion as the hyoid arch is reduced, and the branchial arches are removed ; thus a strong fulcrum is completed for the articulation of the shoulder-joints. The pelvic arch had previously been' completed, and the iliac bones and sides of the sacrum become co-elongated : then the Uia continue to extend backwards as the tail is being absorbed, and the hind-limbs are lengthened out and finished. Thus metamorphosed, the skeleton of the frog presents the following structure (Fig. 12) : — The number of vertebrse of the trunk, exohisive of the coxygeal style, o, is nine ; the first, or atlas, has no diapophyses, but these are present and long on the rest, especially on the third, d, and ninth, «, vertebrae ; in the latter they are thick, stand out- wards, and support two other long, curved, rib-Hke bones, 62, which expand at their distal ends, and unite to two bony plates, 63, completing the ha;mal arch of the ninth segment of the trunk. The bonfs of the hinder extremities are attached to the point of union of the above costal and haemal pieces, one of which answers to the Uium, 62, and the other to the ischium, 63. The superior development of this arch relates to the great size and strength of the hinder extremities in the taU-less tribe. The bodies of th« vertebrse are articulated by ball-and-socket joints, the cup being anterior, the ball posterior, a modification which relates to the more terrestrial habits and locomotion of these )ugher- orgauized batrachia. The caudal vertebrae are represented by a single, elongated, cylindrical style, c, having an anchylosed neural canal. In the seven vertebrae, between the atlas and the sacrum, two zygapopophyses, looking upwards, two zygapophyaes, e, looking downwards, and a short spine,- are developed from each neural arch. The suprascapula, 50, is very broad, and in great part ossified ; the scapula, 51, divides at its humeral end into an acromial and coracoid process ; the latter articulates with the true coracoid bone, 52, the acromion with the expanded extremity of the clavicle, 58 : the glenoid cavity is formed by both the scapula and the coracoid. An epistemal boue,- 59, supporting a broad cartilage, is articulated to the mesial union of the clavicles, from which a bony bar is continued backwards between the expanded and par- tially conjoined ends of the coracoids. The sternum, 60, is articulated to the posterior part of the same extremities of the coracoids, and supports a broad " xiphoid" cartilage. The proximal end of the humerus, 53, is an epiphysis ;■ the distal end presents a hemispherical ball between a small externa! ridgfe,. aiid' a' large internal condyloid process. The antibrachial bones have coalesced^, but an anterior and posterior inden- tation at the distal half indicates the radius, 55; ahd ulna, 54 ; their distal articular extremities are represented by a single epiphysis. The ulnar portion of the bone developes a short and broad olecrauotf, o.- The bones of the carpal series now receive definite names, and are as follows :— -(Fig. 12),*, scaphoid; I, lunare ; c andp, cuneo- pisiforme; <, trapezium-; vo hundred or three hundred vertebrse fit together, that even in the relaxed and deaji state the body cannot be twisted except in a series of side coils. In the construction of the skull, which has merited a description in some detail, and well deserves a close study, the thickness and density pf the cranial bones must strike the miiid as a special provision against fracture and injury to the brain. "When we contemplate the still more remai-kable manner in which these bones ' are applied, one over another, the superocci- Fig. 17. pital (Fig. 17), 3, overlapping the exoccipital, 4, and the parietal, 7, overlapping the superocci- pital, — the natural segments or vertebrae of the cranium being sheathed, one within the other, like the corresponding segments in the trunk, — we oaimpt but discern a special adaptation in the structure of serpents to their ooiamouly prone position, arid ^ provision, exemplified in such structure, of the dangers to which they section op bkhll, boa coHSTEioroa. would be subject from falling bodies and the tread of heavy beasts. Many other ec;ually beautiful instances of design might be cited from the organization of serpej^ts in relation to the necessities of their apodal vermiform character; just as tho snake-like ccl is compensated by analogqus modifications amongst fishes, and the suake,li]fe cen- tipede among insects Osteology qf I,izaids,--The transition irom the ophidian, pr snake-like, to the VERTEBRA AJTD SKULL OP THE LIZAED. 199 lacertian, or lizard-like reptiles, is very gradual and easy, if we pass from the serpents with fixed jaws and a scapular arch — as, e. g., the slowrworms [/ingms) — ^to the ser- pentiform lizards with mere rudiments of limbs — as, e. g., the pseudopus. The distinc- tion is etfected through the establishment of a costal arch in the trunk, completed by the addition of a haemal spine (stemom) and hajmapophyses (sternal ribs) to the pleur- apophyses or vertebral ribs, which are alone ossified in ophidia. The Tertebraa of the trunk have the same procoelian character, i, e, with the cup anterior aaid the ball behind ; the latter being usually less prominent, more oblique, and more transversely oval than in serpents. The vertebrae also are commonly larger, and always fewer in number than in the typical ophidia. The ribs do not begin to be developed so near the head in lizards. Not only the atlas and dentata, but sometimes, as in the monitor [varantia], the four following vertebrae are devoid of pleurapophyses ; and when these first appear they are short, and sometimes (as in cyelodtn) expanded at their extremities. They rapidly elongate in succeeding vertebrae, and usually at the ninth from the head {cgclodus, iguana), or tenth {varanus), they are joined through the medium of ossified haemapophyses to the sternum ; two (fiaranus), three [chameleo, iguana), or four [cyehdus), following vertebrae are similarly completed ; and then the haemapophyses are either united below without intervening sternum (c/mmeleo), or two or tiiree of them are joined by a common cartilage to the oartilagiuous end of the sternum. The haemapophyses afterwards project freely, and are reduced to short appendages to the pleurapophyses. These also shorten, and sometimes suddenly, as, e.g., after the eighteenth vertebra in the monitors {varanm), in which they end at the twenty-eighth vertebra, as they began, viz., in the form of short straight appendages to the dia- pophyses. » The flying lizard {Draco volans), is so called on account of the wing-like expansions from the sides of its body, supported, Hke the hood of the cobra, by slender elongated ribs. In this little lizard there are twenty vertebrae supporting moveable ribs, which commence apparently at the fifth. Those of the eighth vertebra first join the sternum, as do those of the ninth and tenth ; the pleurapophyses of the eleventh vertebra sud- denly acquire extreme length ; those of the five following vertebrae are also long and slender ; they extend outwards and backwards, and support the parachute formed by the broad lateral fold of the abdominal integuments. The pleurapophyses of the seven- teenth vertebra become suddenly shorter, and these elements progressively diminish to the sacrum : this consists of two vertebrae, modified as in other lizards. There are about fifty caudal vertebrae. The semi-ossified sternum in the iguana has a median groove and fissure, and readily separates into two lateral moieties. The long stem of the epistemum covers the outer part of the groove, where it represents the /eeel of the sternum in birds. In the skull of the lizard order we first meet with a second bony bar, diverging from the maxillary arch backwards, and abutting against the mastoid, and sometimes also against the tympanic and postfrontal. This bar is called the " zygomatic arch ;" it usually consists of two bones — ^the one next the maxillary is the "malar," 26, the one next the mastoid is the " squamosal," 27 ; it assumes a form meriting that name in the tortoise, and first received it, as " pars squamosa," in man, where it is not only like a great scale, but becomes confluent with both the mastoid and tympanic. But, as has been before remarked, we must use the terms invented by anthropotomists as arbitrary signs of the corresponding bones in the lower creation. The scapula in the monitor (varanm) is a triangular plate with a convex base, a %. — • ~ 200 OSTEOLOGY OF OROCODlxES. concaTe hind border, and a nearly straight front border ; the apex ia thick and truncate, with an oval surface divided into two facets. The hind border forms h part of the glenoid cavity ; the front one is a rough epiphysial surface, continuous with a similar but narrower tract, extending upon the anterior border, and by which the scapuLd articulates with the coracoid. In the ig-uanians and scincoids this synchondrosis is obliterated, and the two bones are confluent. The hind border of the scapula is nearly straight— the front one sends forwards a process dividing it into two deep marginations. The coracoid in both the varanus and iguana is short and broad ; its main body, which articulates with the sternum, is shaped like an axe-blade, and two strong, straight, compressed processes extend forwards from its neck, which is perforated between the" origius of these processes and the part forming the glenoid articulations. The clavicles are simple sigmoid styles in the varanus and iguana ; are bent upon themselves, like the Australian boomerang, in the oyclodus ; and have the median part of the bend expanded and perforated iu lacerta and sciucus. They are absent in the chameleon. The sacral vertebrae retain, in some lacertians, the cup-and-baU joints ; and in these — e. g., the scincoids— in which the centrums coalesce, the hind end of the second presents a ball to the first caudal — not a cup, as in the crocodile. In the cyclodus the thick, short, straight pleurapophyses are distinct at their origins from the two coalesced centrums, but coalesce at their ends, that of the first sacral being the thickest. In varanus and iguana the pleurapophyses, as well as the centrumsj retain their dis- tinctness, but the hinder ribs incline forwards and touch the expanded ends of th» fore pair. These ends are very thick, and are scooped out obliquely behind, so as to present a curved border to the ilium, which Cuvier compares to a horse-'shoe. In the varanus and iguana the pleurapophyses of the first caudal incline backwards as much as those of the second sacral do forwards. In the cyclodus they extend outwards, parallel with those of the sacral vertebrae, and are longitudinally grooved beneath. Haemapophyses are wanting iu the first caudal, are developed in the second, and are displaced to the interval between this and the third ; they are confluent at their distal end, and produced iato a long spiue. At the twelfth tail-vertebra the line is obvious that indicates the extent of the anterior detached piece, or epiphysis, of the centrum, immediately in front of the origin of the diapophyses ; it continues marking off the anterior third of the centrum in all the other caudals. At this line the tail snaps off, when a lizard escapes by the common ruse of leaving the part of the tail by which it has been seized in the hands of the bafiled pursuer. It is a very curious cha- racter, and quite peculiar to the lacertians — this ossification of the centrum from two points and their incomplete coalescence : it adds nothing to the power of bending or to any other action of the tail, but indicates a prevision of the liability to their being caught by their long taU, and may be interpreted as a provision for their escape. The neural arch has coalesced with the centrum throughout the taU : tlie epiphysial line does not extend through that arch ; but its thin and brittle walls soon break, when the two parts of the centrum are forcibly separated. Lizards, as is well known, have the power of reproducing the tail, but the vertebral axis is never ossified in the new-formed part. Osteology of Ciocodiles.-The numerous and varied forms of fossil bones of extinct reptiles derive most elucidation from the skeleton of the higher organized sauria of Cuvier, which now are rightly held to constitute a distinct order, called Zoricata or Orocoiiliat a more complete description, therefore, will be given of the skeleton of SKELETON OF THE CKOO'ODILB. 2Ul a member of this order than ■was deemed needful in regard to the lacertian group of eauria. _, >...,, a- Ml Fig. 18. — SKELETON OP THE cnocoDiLE [d'OcoMlus tiiloticus) . Kg. 19. ATLAS AND AXIS VERTEBRiE OF THE CEOCODILE. Commencing with the trunk, the first and second vertebrae of the neck are pecu- liarly modified in most air-breatUng vertebrala, and have accordiugly received the special names, the one of "atlas," the other of "axis." In comparative anatomy these become arbitrary terms, the properties being soon lost which suggested those names to the human anatomist; the "atlas," e.^., has no power of rotation upon the " axis," in the crocodile, and it is only in the upright skeleton of man that the large globular head is sustained upon the shoulder-like processes of the " atlas." In the crocodilei these vertebrae are con- cealed by the peculiarly prolonged angle of the lower jaw in the side view of the skeleton (Fig. 18), and a figure of the two vertebrae is therefore subjoined (Fig. 19). The pleurapophyses, pi, are retained in both segments, as in aU the other vertebra3 of the trunk. That of the atlas, pi, a, is a simple slender style, articulated by the head only, to the " hypapo- physis," a%. The neurapophyses, na, of the atlas retain their primitive distinctness ; each rests in part upon the proper body of the atlas, ca, in part upon the hypapophysis. The neural spine, ns, u, is also here an independent part, aud rests upon the upper extremities of the neurapophyses. It is broad and fiat, and prepares us for the further metamorphosis of the corresponding element iu the cranial vertebrae. The centrum of the atlas, ca, called the " odontoid process of the axis" in human anatomy, here supports the abnormaHy-advanced rib of the axis vertebra, ;)Z, «. The proper centrum of the axis vertebra, ox, is the only one in the cervical series which does not support a rib ; it articulates by suture with its neuiapophyses, nx, and is characterized by having its anterior surface flat, and its posterior one convex. "With the exception of the two sacral vertebrae, the bodies of which have one arti- cular surface flat and the other concave, and of the first caudal vertebra, the body of which has both articular surfaces convex, the bodies of aU the vertebrae beyond the axis have the anterior articular surface concave, and tbe posterior one convex, and articulate with one another by baR-and-socket joints. This type of vertebra, which I have termed "procoeKan" (irpos, before, ko.\of, concave), characterizes aU the existing genera and species of the order Crocodilia with all the extinct species of the tertiary periods, and also two extinct species of the green-sand formation in New Jersey.* • " Quarterly Journal of the Geological Society," November, 1849. 202 VEETEBEvE OP THE- CBOCODIIE. Here, so far aa our present knowledge extends, the type was lost, and other dispositions of the articular surfaces of the centrum occur in the vertebrae of the crocodilia of the older secondary formations. The only known crocodilian genus of the periods ante- cedent to the chalk and green-sand deposits with vertebrae articulated together by ball-and-socket joints, have the position of the cup and the balj the reverse of that in the modem crocodiles, and one genus, thus characterized by vertebras of the " opistho- Goeliam" type {ottljBos, behind, koAos, concave), has accordingly been termed strepto- spondylus, signifying " vertebrae reversed." But the most prevalent type of vertebra amongst the crocodilia of the secondary periods was that in which both articular surfaces of the centrum were concave, but in a less degree than in the single concave surface of the vertebrae united by ball and socket. Vertebrae of this " amphioceKan" type {a^if both, koi\os, concave) existed in the teleosaurus and steneosaurus. In the ichthyosaurus the concave sxirfaces are usually deepened to the extent and in the form shown in those of the fish (Cut 8). Some of the most gigantic of the crocodilia of the secondary strata had one end of the vertebral centrum flattened, and the other (hinder) end concave ; this " platycoelian" type (itAbtiii, flat, Kothos, concave) we find in the dorsal and caudal vertebrae of the gigantic cetiosaurus. With a few exceptions, all the modem reptiles of the order lacertUia have the same procceliau type of vertebrae as the moderu crocodilia, and the same structure pre- vailed as far back as the period of the mosasaurus, and in some smaller members of the lajertilian order in the cretaceous and wealdep. epoclis. Eesuming the special description of the osteology of the modem crocodilia, we find the procoeUan type of centrum established in the third cervical, which is shorter but broader than the second ; a pgrapophysis is developed from the side of the centrum, and a diapophysis from the base of the neural arch ; the pleurapophysis is shorter, its fixed extremity is bifid, articulating to the two above-named processes ; its free extremity expands, and its anterior angle is directed forwards to abut against the inner surface of the extremity of the rib of both the axis and atlas, whilst its posterior prolongation overlaps the rib of the fourth vertebra. The same general characters and imbricated coadaptation of the ribs (Fig. 18), pi, characterize the succeeding cervical vertebrae to the seventh inclusive, the hypapophysis progressively thoughth slightly increasing in size. In the eighth cervical the rib becomes elongated and slender ; the anterior angle is almost or quite suppressed, and the posterior one more developed and produced more downwards^'so as to form the body of the rib, which terminates, however, in a free point. In the ninth cervical, the rib is increased in length, but is stiU what would be termed a " false" or "floating rib" in anthropotomy. In the succeeding vertebra the pleurapophysis articulates with a haemapophysis, and the haemal arch is completed by a haemal spine ; and by this completion of the typical segment we distinguish the commencement of the series of dorsal vertebrae (i4.) D. With regard to the so-called " perforation of the transverse process" this equally exists in the present vertebra, as in the cervicals ; on the other hand, the cervical vertebrae equally show surfaces for the articulation of ribs. The typical characters of the seg- ment, due to the completion of both neural and haemal arches, are continued in some species, of crocodilia to the sixteenth, in some [crocodUus acutus) to the eighteenth vertebra. In the croeodilua acutus and the alligator lucius the hcemapophysis of the eighth dorsal rib (seventeenth segment from the head) joins that of the antecedent ver- tebra. The pleurapophysos project ireely outwards, and become " floating ribs" in the eighteenth, nineteenth, and twentieth vertebriE, in which they become rapidly shorter VERTEBRA OF THE CROCODHIA. 203 and in the last appear aa mere, appendages to the end of th.e long and broad diapophyses : but tbe hajmapopiysea by no means disappear after the solution of their union with their pleurapophyses ; they are essentially independent elements of the segment, and they are continued, therefore, in pairs along the ventral surface of the abdomen of the crooodilia, as far as their modified homotyp§s the pubic bones. They are 'more or less ossified, and are generally divided into" two or three pieces, The lumbar vertebrae ajre those in which the diapophys.es ceaae to support moveable pleurapophyses, although they are elongated by the coalesoJft rudiments^ such which are distinct' in the young crocodilia. The length and p^j^istent individuality of more or fewer of these rudimental ribs determines the number of Ute dorsal and lumbaXj^ yertebree respectively, and exemplifies the purely artificial character of the distinction. The number of vertebrae or segments between the skull and the sacrum, in all the croebdilia I have yet examined, is twenty-four. In the skeleton of a gavial I have seen thirteen dorsal and two lumbar; in that of a crocodilus cataphractus twelve dorsal and three lumbar , in those of a cropodilus acutus and alligator lucius, eleven dorsal and four lumbar, and this is the most common number ; but in the skeleton of the crocodile, probably the species called croc, biporcatus, described by Cuvier, he gives five as the number of the lumbar vertebrae. But these varieties in the develop- ment or coalescence of the stunted pleurapophysis are of little essential moment ; and only serve to show the artificial character of the " dorsal" and " lumbar" vertebrae. The coalescence of the rib with the diapophysis obliterates of course' the character of the " costal articular surfaces," which we have seen to be common to both dorsal and cervical vertebrae. The lumbar zygapophyses have their articular surfaces almost hori- zontal, and the diapophyses, if not longer, have their antero-posterior extent somewhat increased ; they are much depressed, or flattened horizontally. The sacral vertebrae are very distinctly marked by the flatness of the coadapted ends of their centrums ; there are never more than two such vertebrae in the crocodUia-tecent or extinct : in the first the anterior surface of the centrum is concave ; in the second it is the posterior surface ; the zygapophyses are not obliterated in either of these sacral vertebrae, so that the aispects of their articular surface— upwards in the anterior pair, downwards, in the posterior pair — determines at once the corresponding extremity of a detached sacral vertebra. The thick and strong transverse processes form another characteristic of these' vertebras ; for a long period the suture near their base remains to show how large a proportion is formed by the pleurapophysis. This element articu- lates more with the centrum than with the diapophysis developed from the neural arch ; it terminates ^y a rough, truncate, expanded extremity, which almost or quite joins that of the similarly but more e^anded rib of the other sacral vertebrae. Against these extremities is applied a, supplementary costal piece, serially homologous with the appendage to the proper pleurapophysis in the dorsal vertebrae, but Here interposing itself between the pleurap^hyses and haemapophyses of both sacral vertebrae, not of one only. This intermediate pleurapophysial appendage is called the " ilium ;" it is short, thick, very broad, and subtriangular, the lower truncated apex forming with the connected extremities of the haemapophysjs an articular cavity for the diverging append- age, called the " hind leg." The haemapophysis of the anterior sacral vertebra is called "pubis," 64 ; it is moderately long and slender, but expanded and flattened at its lower extremity, which is directed forwards towards that of its fellow, and joined to it through the intermedium of a broad, cartilaginous, haemal spine, conlpleting the haemal canal. The posterior hsemapophysigL 63, is broader, subdepressed, and subtriangular, expanding 204 SKULL OF THE CROCODILE. as it approaches its fellow to complete the second heemal arch ; it is termed " ischium. " The great development of all the elements of these haemal arches, and the peculiar and distinctive forms of those that have therehy acquired^ from the earliest dawn of anato- mical science, special names, relates physiologically to the functions of the diverging appendage which is developed into a potent locomotive member. This limb appertains properly, as the proportion contributed by the ischium to the articular socket and the greater breadth of the gleurapophysis showj to the second sacral vertebra ; to which the ilium chiefly belongs. The first caudal ^ertebraj which presents a ball for articulating with a cup on the back part of the last sacral, retains, nevertheless, the typical position of the ball on the back part of the centrum ; it is thus biconvex, and the only vertebra of the series which presents that structure. The first caudal vertebra, moreover, is distinguished from the rest by having no articular surfaces for the hsemapophysesj which in the succeeding caudals form a hsemal arch, like the neurapophyaes above, by articulating directly with the centrum. The arch so formed has its base not applied over the middle of a single centrum, but, like the neural arch in the back of the tortoise and sacrum of the bird, across the interspace between two centrums. The first haemal arch of the tail belongs, however, to the second caudal vertebra, but it is displaced a little backwards from its typical position. The caudal heemapophyses, h h, coalesce at their lower or distal ends, from which a spinous process is prolonged downwards and backwards ; this grows shorter towai'da the end of the tail) but is compressed and somewhat expanded antero-posteriorly. The haemal arch so constituted has received the name of " chevron bone." It is very true, as Cuvier said in the last lectuie he delivered, " if we were agreed as to the crocodile's head, we should be so as to that of other animals ; because the crocodile is intermediate between mammals, birds, and fishes." Accordingly, the follow- ing description of the crocodile's skull is coextensive with that of the fish ; if the answerable bones are rightly determined between these, their correspondence with those of other vertebrates will be facilitated. The difficulties in comprehending the nature of some of the bones of the crocodile's head have arisen through passing to its comparison from that of the mammal's skull^^by descending instead of ascending to it. The segments composing the skull are more modified than those of the pelvis ; but just as the vertebral pattern is bfest preserved in the neural arches of the pdvis, which are called collectively " sacrum," so, also, is it in the same arches of the skuU, which are called collectively " cranium. The elements of which these cranial arches are composed preserve, moreover, their primitive or normal individuality more completely than in any of the vertebrae of the trunk, except the atlas, and consequently the archetypal charaoter can be more completely demonstrated.* If, after separating the atlas from the occiput, we proceed to detach the occipital segment of the cranium from the next segment in advance, we find the detached segment presenting the form and structure of the neural arch. The "centrum" presents like those of the trunk, a convexity or ball at its posterior articular surface, but its anterior one, like the hindmost centrum of the sacrum, unites with the next centrum in advance by a flat rough " sutural " surface. Like most of the centrums in the neck and begin- ning of the back, that of the occiput developea a hypapophysis, but this descending « The skull of tho crocodile; partially disarticulated, and with the bones numbered as in the following description, may be bad of Mr. Flower, No. i!l, Lambeth Terraoe, Lambeth Uoad. SKULL OP THE OKOOODILE. 2Q5 process is longer and larger, its^ase exteiiding over the vjiolo of the under surftce of the centrum. It is a character whereby the occipital centrum of a crocodilian reptile may be distinguished from that of a laoeft^n one ; for ip the latter a pair of diverging hypapophyses project (rom the uiider surface, as is shovii ip moat recent lizards and in the great extinpt mosasaurus. The upper aii4 lateral parts of Nq, 1 present rough autural surfaces, like those in the centrums of the trunk, for articulating -with the <' neurapophyses," Nos. 2, 2, ■which develope short, thick, obtuse, transverse processes, 4, 4. The modified or specialized pharacter of the elements of the cranial vertebrae hag gained for them! special names. The centrum, Jj is calle4, as in ftshes and all other vertebrates, the "basiT occipital ;" the neurapophyses, 2, 2 , are the " ezoccipitals ;" the neural spine, 3, is the " superoccipital ." The transverse processes, 4, 4, which may combine both diapophysea an4 parapophyses, are called the ' ■ paroccipitals ;" they are never detached bones in the crocodilia, as they are in the chelonia and in most fishes. The ezoccipitals perform the usual functions of neurapophyses, and, HJce those of the atlas, meet above the neural canal ; they are perforated to give exit to the vagal apd hypoglossal nerves, and protect the sides of the medulla oblongata and cerebellura — the two divisions of the epence- phalon. The superoccipital, 3, is broad and flat, like the similarly detached neural spine of the atlas ; it advances a little forwards, beyond its sustaining neurapophyses, to protect th,e upper surface of the cerebellum ; it is traversed by tympanic air-cells, and assists with tlie exocoipitals, 2, 2, in the formation of the chamber for the internal car. * The chief modification of the occipital segment of the skull, as compared with that jf the osseous fish, or with tlie typical vertebra, is the absence of an attached hsemal arch. AVe sliaU afterwards see that this arch is present in the crocodile, although displaced backwai^s. Proceeding with the neural arches of the crocodile's skull, if we dislocate the segment in advance of the occiput, we bring away, in connection with the long base- ■* bone, 5, the hone, 9, which in the figure of the section of the serpent's skuU (Cut 17) is shown sinjilarly united to 6. In fact, the centrums of the vertebrae have here coalesced, as we find to happen in the neofe of the siluroid fishes, and in the sacrum of birds and mammals. The two connate cranial centruins must be artificially divided, in order to obtain the segments distinct to which they belong. The hinder portion, 5, of the great tase-bone, which is the centrum of the parietal vertebra, is called " basisphenoid ." It supports that part of the " mesencephalon," which is formed by the lobe of the third ventricle, and its upper surface is excavated for the pituitary prolongation |of that cavity. The basisphenoid developes from its under surfape a " hypapophysia," which is suturally united with the fore part of that of the basioccipital, but extends further down, and is similarly united in front to the "pterygoids," 24 . These rough sutural surfaces of the long desceRding process of the basisphenoid are very characteristic of that centrum, when detached, in a fossil state. The neurapophyses of the parietal ver- tebra, 6^ 6, or the " alfephenoids ," protect the sides of the mesencephalon, and are notched at their anterior margin, for a conjugational foramen transmitting the trigeminal nerve. As accessory funotio& they contribute, like the corresponding bones in fishes, to the formation of the ear-chamber. They have, however, a little retrograded in position, resting below in part upon the occipital centrum, and supporting more of the spine of that segment, 3, than of their own, 7. The spine of thejj^etal vertebra is a per- manently distinct, single, depressed bone, like that of the occipital vertebra ; it is called -i^ 206 SKULL OF THE CROCODILE. the "parietal, " iand completes the neuial arch, as its crown or key-bone ; it is partiaL.,, excayated by the tympanic air-cells, and overlaps the superoccipital. The bones, 8, S. wedged between 6 and 7, manifest more of their diapophyaial character than their homotypes, 4, 4, do in the occipita;! segment, since they support modified ribs, are developed &om independent centres, and preserve their individuality. They form no part of the inner walls of the craniilm, but §6nd outwards and backwards a strong transverse process for muscular attachment. They afford a ligamentoiis attachmast to the hs^Ynal arch of their own segment, and articulate largely with the pleurapo- physes, 28, of the antecedent haemal arch, whose more backward displacement, in comparison with its position- ill the fish's skull, is well illustrated in the metamorphosis of the toad and &og. On removing the neural arch of the parietal vertebra, after the sectioft of its 6onfluent centrum, the elements of the corresponding arch of the frontal vertebra present the same arralrgemeit. The compressed produced centrum has its form modified like that of the vertebral centnuns at the opposite extreme of the body ill many birds ; it is called the " prespheioid." The neurapophyses, 10, 10, articulate with the upper part of 9 ; they are exp&tfded, and smoothly excavated on their inner surface to support the sides of the large' prosencephalon ; they dismiss the great optic nerves by a notch. They show the ssime tendency to a retrograde change of position as the neighbouring neurapophyses, 6 ; for though they support a greater proportion of their proper spine, 1 1, they also support part of the parietal spine, 7, and rest, in part, below upon the parietal centrum, 5 : the neurapophyses, 10, 10, are called " orBitosphenoids ." The ngural spine, 11, of the frontal vertebra retains its normal character as a sii^le symmetrical bone, Uie the parietal spine which it partly overlaps ; it also completes the neural arch of its own segment, but is remarkably extended longitudinally forwards, where it is much thickened, and assists in forming the cavities for the eye-bfflls ; it is oaSted the "frontal/' bone. In contemplating in the skull itself, or such side view as is given in Fig. 9, p, 22, of my -frork on the Archetype Skeleton, the relative position of the frontal-, 11, to the parietal, 7, and of this to the superoccipital, 3, which is overlapped by the parietal, just as itself overlaps the flattened spine of the atlas. We gain- a con- viction which cannot be shaken by any difference in theii' mode of ossification, by their median bipartition, or by their extreme expansion in other animals, that the above-named single, median, imbricated bones, each complteting its neural aich and permanently distinct from the piei^ of such arch, must repeat the same element in those successive arches — in other *-ords, must be "homotypes," or serially homoloo-ous fn like manner the serial homology of those piers, called "neurapophyses," viz., the laminae of the atlas, the exoccipitals, the alisphenoida, and the orbitosphenoids is equally immistakable. Nor can we shut out of view the same serial relationship of the paroccipitals, as coalesced diaphophyses of the occipital Vertebra, -with the mastoids 8, and the postfrontals, 12, as permanently detached diapophyses of theii' respective vertebrae. All stand out from the sides of the cranium, as tranvette processes for mus- cular attachment ; all are alike autogenous in the turtles ; and all of them, in fishes offer articular surfaces for the ribs or haemal arches of their respectivlVertebta • and these characters are retained in the postfrontals as well' as in the mastoids of the crocodiles The frontal diapo]»hysis, 12, is wedged between the back part of the spine, H and the neurapophysis, 10 ; iif outwardly projecting process extends also backwards, and joins that of the succeeding diapophysis, 8 ;■ but, notwithstanding the retrogradation of SKULL OF IHB CEOCOtttLfi. 207 the inferior arch, it still articulates witli part of its own plenrapopbysial element, 28, which forms the proximal element of that arch. There finally remain in the cranium of the crocodile, after the successive detach- ment of the foregoing arches, the bones terminating the fore-part of the skull ; but, notwithstanding the extreme degree of modification to which their extreme position subjects them, we can stiU trace in their arrangement a correspondence with the yer- tebrate type. illong and slender symmetrical grooved bone, 13, between 24 and 24, like the ossified inferior haH of the capsule of the notochord, is continued forwards from the inferior part of the centrumj 9, of the frontal vertebra, and stands in the relation of a centrum to the vertical plates of bone, 14, which expand as they rise into a broad, thick, trian- gular plate, with an exposed horizontal superior surface. These bones, which are called " prefontals, " stand in the relation of "neurapophyses" to the rhineucephalic pro- longations of the brain commonly but erroneously called ." olfactory nerves ;" and th^y form the piers or haunches of a neural arcb, which is completed above by a pair of symmetrical bones, 15, called " nasals," which I regard as a divided or bifid neural spine. The centrum of this arch is established by ossification in the expanded anterior prolongation of the fibrous capsule of the notochord, beyond the termination of its gelatinous axis. The median portion above specified retains most of the formal characters of the centrum ; but there is a pair of long, slender, symmetrical ossicles, which, from the seat of their original development, and their relative position to the ■neural arch, must be regarded as also parts of its centrum. And this ossification of the element in question from different centres wHl be no new or strange character to those who recollect that the vertebral body in man and mammalia is developed from three centres. The term. " vomer " is applied to the pair of bones, 13, because their special homology with the single median bone, so called in fishes and mammals, is indisputable ; but a portion of the same element of the skull retains its single symmetrical character in the crocodile, and is connate with the enormous pterygoids, 24, between which it is wedged. In some alligators (all. niger) the divided anterior vomer extends far forwards, expands anteriorly, and appears upon the bony palate. Almost all the other bones of the head of the crocodBe are adjusted so as to constitute four inverted arches. These are the haemal arches of the four segments or vertebrae, of which the neural arches have been just described. But they have been the seat of much greater modifications, by which they are made subservient to a variety of function^ unknown in the haemal arches of the rest of the body. Thus the two anterior haemal arches of the head perform the oflice of seizing and bruising the food; are armed for that purpose with teeth : and, whilst one arch is firmly fixed, the other works upon it Eke the hammer upon the aftvil-. The elements of the fixed arch, called " maxiUapj arch," have accordingly undergone the greatest amount of morphological change, in order tO' adapt that arch to its shale in mastication, as well as for forming part of the passage &r the respiratory medium, which is perpetually traversing this haemal canal in its way to purify the blood. Almost the whole of the upper surface of the maxillary' arch is firmly united, to contiguous parts of the skull by roiigh or sutural surfaces, and its strength is increased by bony appendages, which diverge from it to abut against other parts of, the skuU. Comparative anatomy teaches that, of the numerous places of attachment, the one which connects the maxillary arch by its element, 20, with the centrum,, 13, and the descending plates of the neurapophyses, 14, of the nasal segment. 208 EKTTLL OF THE CROCODILE, is the normal or tjie most constant point of its BuspefEion, the bone, 20, being the pleuiapophysial element of the maxillpiy ^rch: it is called the " palatine, " because the xmder surface forms a portion of the bony roof of the mouth, called the " palate." It is articulated at its fore part with the bone, 21, in the same plates, which bone is the hsemapophysial element of the maxillary arch : it is caUed the '* manllary," and is greatly developed both in length and breadth ; it is connected not only with 20 behind, and 22 in front, which are parts of the same arch, and with tJie diverging appendages of the arch, viz., 26, the malar bon e, and 24, the pter ygoid, but ajso with the nasals, 15, and the lacrymal, 16, as well as with its fellow of the opposite side of the arch. The smooth, expanded horizontal plate, wjiich effects the latter junction, is called the palatal plate of tbe maxiUary ; the thickened external border, where this plate meets the external rough surface of the bone, and which is perforated for the lodgment of the teeth, is tbe " alveolar border" or "process" of t)ie maxUlary. The haemal spine or jl^^bone of the arch, 22, is bifid, and the arch is completed by the symphysial junction of the two symmetrical halves ; these halves are called " premaxillary bones :" these bones, like the maxillaries, bate a rough facial plate, and a smooth palatal plate, with the connecting alveolar border. The median symphysis is perforated vertically through both plates ; the outer or upper hole being the external nostrE, the under or palatal one being the prepalatal or nasorpalatal aperture. Both the palatine and the maxillary hones send outwards and backwards parts or processes which diverge from the line of the l^semal arch, of which they are the chief elements ; and these parts give attachment to distinct bones which form the " diverging appendages" of the arch, and servo to attach it, as do the diverging appendages of the thoracic haemal arches in the bii-d, to the succeeding arch. The appendage, 24, called "pterygoid," effects a more extensive attachment, and is peculiarly developed in the crocodiUa. As it extends backward it expand, vinites with its fellow below the nasal canal, and encompassing that canal, coalesces above it with the vomer, and is firmly attached by suture to the presphenoid and basisphenoid : it surrounds the hinder or palatal nostril, and, extending outwards, it gives attachment to it second bone, 25, called " ectopterygoid ," which is firmly connected with the maxfllary, 2^, the malar, 26, and the post-frontal, 12. The second diverging ray is of great strength ; it extends from the maxiUary, 21 (" haemapophysis " of the maxillary arch), to the tympanic, 28 (" pleurapophyses" of the mandibular arch), and is divided into two pfeces, the malar, 26, and the squamosal, 2 7. Such are the chief crocodilian modiflations of the haemal arch, and appendages of the anterior or nasal vertebra of the skull. The haemal arch of the frontal vertebra is somewbat less metamorphosed, and has no diverging appendage. It is slightly displaced backwards, and is articulated by only a small proportion of its pleurapophysis, 28, to the parapopbysis, 12, of its own segment ; the major part of that short and strong rib articulating witji the parapopbysis, 8, of the succeeding segment. The bone, 28, called " tympanic," because it serves to support the " drum of the ear" in air-breathing vertebrates, is short, strong, and immoveably wedged, in the crocodilia, between the paroooipital, 4, mastoid, 8, post-frontal, 12 and squamosal, 27 ; and the conditions of this fixation of the pleurapophysis are exemplified in the great development of the haemapophysis (mandible), which is here uni^suaUy long, supports numerous ■ teeth, and requires, therefore, a firm point of suspension, in the violent actions to whiph the jaws are put in retaining and overcoming the struggles of a powerful living prey. The moveable articulation between the pleurapophysis, 28, __,_, _ m SKELETOir OP THE CROCODILE. 209 and the rest ot tlie haeinal arch is analogous to that which we find between the thoracic pleurapophysis and hsemapophysis in the ostrich and many other birds. But the hsemopophyais of the mandibular arch in the crocodiles ia subdivided into several pieces, in order to combine the greatest elasticity and strength with a not excessive weight of bone. The different pieces of this purposely subdivided element have received definite names. That numbered 29, which offers the articular concavity to the convex condyle of the tympanic, 28, is called the "articular" piece ; that beneath it, 30, which developes the angle of the jaw, when this projects, is the " angular" piece ; the piece above, 29', is the " surangolar ;" the thin, broad, flat piece, 31, applied, Kke a splint, to the inner side of the other parts of the mandible, is the " splenial ;" the small accessory ossicle, 31', is the " coronoid ," because it developes the process, so called, in lizards ; the anterior piece, 32, which supports the teeth, is called the " dentary ." This latter is the homotype of the premaxillary, or it represents that bone in the man- dibular arch, of which it may be regarded as the hoemal spine ; the other pieces are subdivisions of the hsemapophysial element. The piuport of this subdivision of the lower jaw-bone has been well explained by Conybeare * and Buckland,t by tlie analogy of its structure to that adopted in binding together several parallel plates of elastic wood or steel to malce a crossbow, and also in setting together thin plates of steel in the springs of carriages. Dr, Buckland adds — " Those who have witnessed the shock given to the head of a crocodile by the act of snapping together its thin long jaws, must have seen how liable to fracture the lower jaw would be, were it composed of one bone only on each side." The same reasoning applies to the composite structure of the long tympanic pedicle in fishes. In each case the splicing and bracing togetber of thin flat bones of unequal length and of varying thickness, afibrds compensation for the weakness and risk of fracture that would otherwise have attended the elongation of the parts. In the abdomen of the crocodile the analogous subdivision of the hsemapophyses, there called abdominal ribs, aUows of a alight change of their length, in the expansion and contraction of the walls of that cavity ; and since amphjbious reptiles, when on land, rest the whole weight of the abdomen directly upon the ground, the necessity of the modification for diminished liability to fracture further appears. These analogies are important, as demonstrating that the general homology of the elements of a natural segment of the skeleton is not affected or obscured by their subdivision for a special end. Now this purposive modification of the hsemapophyses of the frontal vertebra is but a repetition of that which affects the same elements in the abdominal vertebrae. Passing next to the hsemal arch of the parietal vertebra, we are first struck by its small relative size. Its restricted functions have not required jt to grow in proportion with the other arches, and it consequently retains much of its embryonic dimensions. It consists of a ligamentous " stylohyal ," its pleurapophysis retaining the same primitive histological condition which obstructs the ordinary recognition of the same elements of the lumbar haemal arches. . A cartilaginous " epibyal," 39 , intervenes between this and the ossified " haemapophysia," 40, which beara the special name of ceratohyal . The haemal apine, 41, retaina its cartilaginous state, like its homotypes, in tbe abdomen; there they get the special name of "abdominal sternum," here of '^ basihyal ." The basihyal has, however, coalesced with the thyrohyals to form a broad OfirtilaginQus plate, the anterior border rising like a valve to close tbe fauces, and the » "Geol. Trans.," 1821, p. 565. + "Bridjewater Treatise," 1838, vol. i., p. 176. 210 IIMES OF THE CEOCODIIifei posterior angles extending beyond and sustaining the thyroid and qthet parts of the larynx. The long hony "coratohyal" and the commonly cartilaginous "epihyal" are suspended by the ligamentotis " stylohyal" to the paroccipital process ; the Whole arch having, like the Inandibular one, retrograded from the connection it presents in fishes. This retrogradation is stUl more considerable ih the succeediig haemal arch. In comparing the occipital segment of the crocodile's skeleton with that of the fish, the chief modificatioh that distinguishes that segment in the crocodile is the apparent absence of its haemal arch. We recognise, hotrever, the special homologues of the constituents of that arch of the fish's skeleton in the bones 51 and 52 of the crocodile's skeleton (Fig. 18) ; but the upper or suprascapular piece, 50, retains, in connection with the loss of its proximal or cranial articulations, its carti lagi nous state ; the scapula, 61, is ossified, as is likewise the corac oid, 52, the lower end of which is separated from its fellow by the interposition of a median, symmetrical, p artiaUy-ossified piece called " epistemum ." The power of recognising the special homologies of 50, 51, and 52 in the crocodile, with the simflarly-uumbered constituents of the same arch in fishes — though masked, not only by modifications of fortn and proportion, hut eren of very* ' substance, as in the case of 50 — depends upon the circumstance of these bones consti- tuting the same essential element of the arclictypal skeleton, viz., tie fourth haemal arch, numbered pi, 52, in Fig. 7 : for although in the present instance there is super- added, to the adaptive modifications above cited, the rarer one of altered connections, Cuvier does not hesitate to give the same names, " sujn-ascapulaire " to 50, and " scapu- laire" to 51, in both fish and crocodile; but he did not perceive or admit that the narrower relations of special homology were a result of, and necessarily included in, the wider law of general homology. According to the latter law, we discern in 50 and 51 a compound " pleurapophysis," in 52 a " hsemapophysis," and in As, the " hsemal spine," completing the hsemal arch. The scapulo-coracoid arch, both elements, 51, 52, of which retain the form of strong and thick vertebral and sternal ribs in the crocodile, is applied in the skeleton of that animal over the anterior thoracic hBemal arches. VieVed as a more robust hsemal arch, it is obviously out of place in reference to the rest of its vertebral segment. If we seek to determine that segment by the mode in which we restore to their centrums the less displaced neural arches of the antecedent vertebrae of the cranium or in the sacrum of the bird,* we proceed to examine the vertebrae before and behind the displaced arch, with the view to discover the one which needs it, in order to be made typically com- plete. Finding no centnim and neural arch without its plenrapophyses from the scapula to the pelvis, we give up our search in that direction ; and in the opposite direction we find no vertebra without its ribs, until we reach the occiput ; there we have centrum and neural arch, with coalesced parapophyses, but without the hfemal arch, which arch can only be supplied by a restoration of the bones 50-52 to the place which they natm-aUy occupy in the slseleton of the fish. And since anatomists are generally agreed to regard the bones 60-52 in the crocodile (Fig. 18) as specially homolo- gous with those so numbered in tie fish (Fig. 9), we must conclude that they are like- wise homologous in a higher sense ; that in the fish the soapula-coracoid arch is in its natural or typical position, whereas in the crocodile it has been displaced for a special purpose. Thus, agreeably with a general principle, we perceive that, as the lower • See " On the Archetype and Homologies of the Vertebrate Skeleton," pp. 117 and 1S9. FOBE-LIMB or THE CKOCODILB. 211 — f ' Tertebrate animal illustrates tie closer adlicsion to the archetype by the natural articu- lation of the scapulo-eoracoid arch to the occiput, so the higher vertebrate manifests tie superior influence of the antagonizing power of adaptive modification by the remova' of that arch from its proper segment. The anthropotomist, by his mode of counting and defining the dorsal vertebrse and ribs, admits, uncouscioutly perhaps, the important principle in general homology which is here exemplified ; and wMch, pursued to its legitimate consequences, and further applied, demonstrates that the scapula is the modified rib of that centrum and neural arch, which he calls the " occipital bone ;" and that the change of place which chiefly masts that relation (for a very elementary acquaintance with comparative anatomy shows how little mere form and proportion affect the homologioal characters of bones), differs only in extent, and not in kind, from the modification' which maies a minor amount of comparative observation requisite, in order to determine the relation of the shifted dorsal rib to its proper centrum in the human skeleton. With reference, therefore, to the occipital vertebra of the crocodile, if the com- paratively well-developed and permanently-distinct ribs of all the cervical vertebrse prove the scapular arch to belong to none of those segments, and if that haemal arch be required to complete the occipital segment, which it actually does complete in fishes, then the same conclusion must apply to the same arch in other animals, up to man himself. The anterior locomotive extremity is the diverging appendage of the arch, under one of its numerous modes and grades of development. The proximal element of this appendage, or that nearest the arch, is called the ",humerus," 53 (Fig. 18). The second segment of the limb consists of two bones ; the larger one, 54, is called the "ulna :" it articulates with the outer condyle of the humerus by an oval facet, the thick convex border of which swells a little out behind, and forms a kind of rudimental " oleoranon ;" the distal eBi is much less than the proximal one, and is most "produced at the radial side. The radius, 55, has an oval head ; its shaft is cylindrical ; its distal end oblong and subcompressed. The small bones, 56, which intervene between these and the row of five longer bones, are called " carpals ; " they are four in number in the crocodilia. One seems to be a continuation of the radius, another of the ulna ; these two are the principal carpals ; they are compressed in the middle, and expanded at their two extremities : that on the radial side of the wrist is the largest. A third small ossicle projects slightly backwards from the proximal end of the ulnar metacarpal ; it answers to the bone called " pisiforme" in the human wrist. The fourth ossicle is interposed between the ulnar carpal and the metacarpals of the three ulnar digits. These five terminal-jointed rays of the appendage are counted from the radial to the ulnar side, and have received special names ; the first is called "poUex," the second " index," the third " medius, '[ the fourth " a nnularis ," and the fifth " minimus ." The firsTjoint of each digit is called " metacarpal ;" the others are termed " phalanx .." In the crocodilia the poUex has two phalanges, the index three, the niedius four, the annu- laris four, and the minimus three. The terminal phalanges, which are modified to support claws, ai-e called " ungual" phalan ges. As the, above-described bones of the scapular extremity are developments of the appendage of the scapular arch, which is the haemal arch of the occipital vertebra, it follows that, like the branchiostegal rays and opercular bones in fishes, they are eSsen- ^ _ ^ — . — 212 PELVIS ANB HIND-LIMB Of THE CaiOCODILr, - ^^_ _ tially bones of tlie head. But the enumeration of the bones of. the CTOCodile's 6kull ia not completed by these ; there is a bone anterior to the orbit, ■which is perforated at its orbital border by the duct of the laorymal gland, -whence it is termed the " lacrymal ( bone, " and its facial part extends forwards between the bones marked 14, 15, 21, and 26. In many crocodHia the^e is a bone at the upper border of the orbit, which extends into the substance of the upper eye^d; it is called " superorbita l." In the crocodilus palpebrosus there are two of these ossicles. Bqth the lacrymal and supero^rlsital bones answer to a series of bones found com- monly in fishes, and called "suborbitals" and " superorbitals." The lacrymal is the most anterior of the sul^orbital series, and is the lai-gest in fishes ; it is also the most constant In the vertebrate series, an^ is grooved or perforated by a mucous duct. These ossicles appertain to the dermal or mucordermal system or " exoskeleton," not to the vertebral system or " endoskeleton." There remains, to complete this slcetch of the osteO|logy of the crocodile, a brief notice of the bones composing the diverging appendage of the pelvic arch : these being a repe- tition of the same element as the appendage of tjie scapular arch, modified and developed for a similar office, manifest a very close resemblance to it. The first bone, called the " femur, " is longer than the humerus, and, like it, prese^its an enlargement of both extremities, w^th a double curvature of the interveni;ig shaft, ^mt the directions are the reverse of those of the humerus, as may be seen in Fig. 18, where the upper or proximal half of the femur is cpneave, ^n^ the distal half convex, anteriorly. The head of the femur is compressed from side to side, not from befpre backwards as in the humerus ; a pyramidal protuberance from the inner surface of its uppgr fourth represents a " tro-, chanter ;" the distal end is expanded transversely, and divided at its back part into two condyles. The next segment of the hind-limb or " leg," includes, like the corres- ponding segment of the fore-limb called " forCTarm," two bones. The largest of these is the "tibia," 66 , and answers to the radius. It presents^ large, triangular head to femur, it terminates below by an oblique crescent with a conve.^i: surface. The "fibula" is much compressed above ; its sh^ft is slender and cyli^^drical, its lower end is enlarged and triangular. The group of small bones which succeed those of the leg are the tarsals ; they are four in number, and have eacl^ a special name. The " astra- galus" articulates with the tibia, and supports the first and part of the second toe. The calcaneum intervenes between the fibula and the ossicle supporting the two outer toes • it has a short but strong posterior tuberosity. The ossicle referred to represents tlie bone called "cuboid" in the human tarsus. A smaller ossicle, wedged between tiv astragalus and the metatarsals of the second and third toes is the " eotoeuneiform " Four toes only are normally developed in the hind-foot of the crocodUia ■ the fifth is represented by a stunted rudiment of its metatarsal, which is articulated to the cuboid and to the base of the fourth metatarsal. The four normal metatarsals are much lonaer than the corresponding metacarpals. That of the first or innermost toe is the shortest and strongest; it supports two phalanges. The other three metatarsals are of neajjv equal length, but progressively diminish in thickness from the second to the fourth ITie second metatarsal supports three phalanges ; the third four ; and the fourth al has four phalanges, but does not support a claw. The fifth digit is represented bv rudiment of its metatarsal in the form of a flattened triangular plate of bone atta b rt to the outer side of the cuboid, and slightly curved at its pointed and prominent end The forms and proportions of the entire skeleton of the crocodile are adapted to th necessities of an amphibious animal, but minister to much more rapid and enerirof OSTBOLOGT OP CHELONIAN REPTILES. 213 movements in water than on laiid. The short limhs preclude the possibility of very quick course along shore ; and the overlapping of the ribs of the neck, whilst enabling the head the better to cleave the water during the acts of diving or swimming, makes the bendiilg Of that part from side to side an act of difficulty and time ; this, it ia said, may avail any one pursued by a crocodile on dry land to escape by turning out of the straight course. But the crocodile usually seizes his prey by Stratagem or concealment when in or cjf se to the water ; and it is there that he shows hiinself master of his position, and chiefly by the powerful strokes of his long, large, vertically-flattened tail. Osteology of Chelonian Reptiles— ToirtoiSes and Turtles.— Those ani- luals to which, in the manifold modifications of the oi^;anic framework, a J)Ortabte dwelling or place of refuge has been given, in compensation for inferior powers of loco- motion or other means of escape or defence, have always attracted especial attention ; and of them the most remarkable, both for the complex constructioti of their abode as well as for their comparatively high organization, are the reptiles of the tohelonian order. The expanded thoracio-abdomiual case, into which, ia. most chelonians, the head, the tail, and the four extremities can be withdrawn, and in some of the species be there shut up by moveable doors closely fitting both the anteribr and posterior apertures -^as, e.ff., in the box-tortoises (cinostemon, cistudo) — ^has been the subject of many and excellent investigations ; and not thte least interesting result has been the discovery that this seemingly special and anomalous superaddition to the ordinary vertebrate structure is due, in a great degree, to the modification of form and size, and, in a less degree, to a change of relative position, of ordinary elements of the vertebrate skeleton. The faat^al dVeHing-chainber of the chclonia consists chiefiy, and in the marihe species {ehelone) and mud-turtles (irioHyx) solely, of the floor and the roof : side- walls Fig. 20. — SKELETON OP THE EUROPEAN TOttTOISE. of variable extent are added in the fresh-water species [emydicms) and land-tortoisca (festudmiana). The whole consists chiefly of osseous "plates" with sunerincumbent homy plates or " scutes," except in the soft or mud-tortoises {trionyx and tpliargu), in which these latter are wanting. 214 CAKAPACB OF THE TTTRTLB. Fig. 20 shows the maimer in which the head and tail can be retracted within the thoracic-abdominal box : the four limbs are figured as extended in the act of walking, to show their structure. The only moreable vertebrEe are those of the neck and tail, and the former enjoy a great degree of flexibility. The vertebrse answering to the dor- sal, lumbar, and sacral series are firmly iixed together ; but the dorsal ones, 1 to 8, are chiefly concerned in the formation of the osseous dwelling-chamber. The composition of this win be first described as it exists in the turtle (c/wlone), the species called " loggerhead" being here selected for its iUuatration. In the marine species of the ehelonian order, of which this may be regarded as the type, the ossification of the carapace and plastron is less extensive, and the whole j( skeleton is lighter, than in the box- Fig. 21. tortoise (Fig. 20), or any of those spe- cies that live on dry land. The head is proportionally larger, — a character common to aquatic animals ; and, being incapable of retraction within the cara- pace, ossification extends in the direc- tion of the fascia covwing the tempo- ral muscles, and forms a second bony covering of the cranial cavity : this accessory defence is not due to the in- tercalation of any new bones, but to exogenous growths from the frontals, 11, postfrontals, 12, parietals, 7, and mastoids, 8. The caiapaee (Fig. 21) is composed of a series of median and symmetrical pieces ch, si to sll, and of two series of unsymmetrical pieces on each side. The median pieces have been regarded as lateral expansions of the summits of the neural spines ; the medio-Iateral pieces as similar developments of the ribs ; and the marginal pieces as the homologues of the sternal ribs. But the development of the carapace shows that ossification begins independently in a fibro-cartUagiuous matrix of the corium in the first, c/i, and some of the last, «9tosll, median plates, and extends from the summits of the neural spines into only eight of the intervening plates, si to «8 : ossification also extends into the contiguous lateral plates, pll topi 8, in some chelonia, not from the corresponding part of the subjacent ribs, but from points alternately nearer and farther from their heads showing that such extension of ossification into the corium is not a development of the tubercle of the rib, as has been supposed. Ossification commences independently in tha corium in all the marginal plates, ml topi/, which never coalesce with the bones uniting the sternum with the vertebral ribs, and which are often more numerous and sometimes leas numerous than those ribs, and in a few species are wanting. Whence it is to be inferred that the expanded bones of the carapace, which are supported and impressed by the thick epidermal scutes called " tortoise-shell," are dermal ossifica- tions, homologous with those which support the nuchal and dorsal epidermal scutes in the crocodile. Most of the pieces of the carapace being directly continuous or connate cvnAPACn OP ruuT.E {tt^e^one imh^afa). PLASTKON OF THE TURTIE. 215 with, the ohTious elements of the vertebrse, which have been supposed exclusiyely to form them by their unusual expausiou, the median ones, si to s 11, have been called "neural plijtes," and the mediorlateral pieces, jiM to^?8, " costal plates ;" but the external lateral pieces, ml to m 12, have retained the name of " marginal plates." The first or anterior of the median plates (cA, " nuchal plate") is remarkable for its great breadth ia the turtles, and usually sends down a ridge from the middle line of its imder surface, which articulates more or less directly with the summit of the neural arch of the first dorsal vertebra ; the second neural plate is much narrower, and is connate with the summit of the neural spine of the second dorsal vertebra : the seven succeed- ing neural plates have the same relations with the succeeding neural spines : the rest are independent dermal bones. The costal plates of the carapace are superadditions to eight pairs of the pleurapophyses or vertebral portions of the second to the ninth ribs inclusive. The slender or proper portions of these ribs project freely for some distance beyond the connate dermal portions, along the under surface of which the rib may be traced, of its ordinary breadth to near the head, which liberates itself from the costal plate to articulate to the interspace of the two contiguous vertebrae, to the posterior of which such rib properly belongs. The plastron, or floor of the bony house, consists in the genus Chelone, as in the rest of the order, of nine pieces, — one median and symmetrical, and the rest in pairs. With regard to the homology of these bones, three explanations may be given : *'^^- '^^• one in conformity with the structure of the thoracic-abdominal cage in the croco- dile ; the other based upon the analogy of that part in the bird ; and the third agreeably with the phenomena of de- velopment. According to the first, the median piece of the plastron, called " ento-stemal," S, answers to the sternum of the crocodile, or " sternum proper," and the four pairs of plastron-pieces, es, hs, ps, xs, answer to the " hoemapophy- ses" forming the so-called sternal and abdominal ribs of the crocodile. Most comparative anatomists have, however, adopted the views of Geoffiroy St. HUaire, who was guided in his determination of the pieces of the plastron by the analogy of the skeleton of the bird ; according to which all the parts of the plastron are re- ferred to a complex and greatly developed sternum, and the marginal plates are viewed as sternal ribs (hsemapophyses). The third ground of determination refers the parts of the plastron, like those of the carapace, to a combination of parts of the endoskeleton with those of the exoskeleton. In Fig. 21, the marginal plates, ml to ml2, are twenty-four in nimibor, or twenty- six if the first (nuchal, ch) and last (pygal, py) vertebral plates he included. Omitting these in the enumeration, three marginal pieces intervene on each side at the angles between the first median plate and the point of the first costal plate formed by the end of PLASTRON OF CHELONE CAOITANNA. 216 veetbbkjE of the toetoise. 8K0UBNT or CARAPACE AND FLASTIION. the seeoncl dorsal rib, which point enters a depression in the fourth marginal piece, m 4 ; the fifth, sixth, seventh, eighth, ninth, and tenth marginal plates are similarly articu- lated by gomphosis to the six succeediug ribs; the eleventh marginal plate has no corresponding rib ; the twelfth is articulated with the point of the ninth dorsal rib sup- porting the eighth costal plate. The want of concordance with the vertebral ribsj or " pleiirapophyses," arising &om the increased number of the mar- F'?- 23. ginal pieces, favours the idea of their being dermal ossifications, such peripheral elements being more subject to vegetative division and multiplication than the hsema- pophyses : the absence of the mar- ginal pieces in the trionyx gives additional support to the same view. The median piece, S, is here regarded as a hsemal spine : it is called " entostemum." Theparial pieces of the plastron axe the " hsemapophyscs " connate with expanded dermal ossifications, and have received the following special names : es, " epistemal ; " " hs, "hyostemal;" jos, " hypostemal ;" xs, "xiphisternal." In some extinct chelonia the number of these lateral elements of the plastron is increased by an intercalated pair which I have called " mesostemals." In the figure of the segment, as modified to form the carapace and plastron (Cut 23)j the nature of the bones is indicated by the letters according to the explanation given of the archetype vertebrsB (Fig. 5, p. 169), the dermal superadditions being marked se. In the figure of the skeleton of the box-tortoise (Fig. 20) a section of the carapace and plastron has been removed from the right side to expose the dorsal and sacral rertebraj, and the disposition of the scapular and pelvic arches^ The eight cervical vertebra? are Iree, moveable, and ribless ; the fourth of these vertebrae has a much elongated centrum, which is convex at both ends ; the eighth is short and broad, vrith the anterior surface of the body divided into two transversely elongated convexities, and the posterior part of the body forming a single convex surface divided into two lateral facets ; the under part of the centrum is carinate. The neural arch, which ia ancliylosed to this centrum, is short, broad, obtuse, and overarched by the broad expanded nuchal plate, ch. The first dorsal vertebra, d 1, is also short and broad, with two short and thick pleurapophyses, articulated by one end to the expanded anterior part of the centrum, and united by suture at the other end to the succeeding pair of ribs. The head of each rib of the second pair is supported upon a strong trihedral neck, and articulated to the interspace of the first and second dorsal vertebrae : it is connate, at the part corresponding to the tubercle, with the first broad costal plate, which arti- culates by suture to the lateral margin of the first neural plate, and to portions of the nuchal and third neural plates : the connate rib, which is almost lost in the substance of the costal plate, is continued with it to the anterior and outer part of the carapace, where it resumes its subcyUudrical form, and articulates with the second and third SKtTLL OP THE T0RT0IS3S. 217 taarginal pieces of the carapace. The neural arch of thu Eeoond dorsal vertebra is shifted forwards to the interspace between its own centi'um and that of the first dorsal vertebra. A similar disposition of the neural arch and spine and of the ribs prevails in the third to the ninth dorsal vertebrae inclusive. The corresponding seven neural plates are,connate with the spines of those vertebrae, and form the major part of the median pieces of the carapace ; the corresponding costal plates, anchylosed to the ribs, form the medio-lateral pieces ; the ninth, tenth, and pygal plates, with the marginal plates of the carapace, do not coalesce with any parts of the endo-skeleton. The bony floor of the great abdominal box; or "plastronj" is formed by the haemapophyses and sternum connate with dermal osseous platesj forming, as in the turtle, nine pieces, one median and symmetrical, answering to the proper sternum, and eight in pairs : but they are more ossified, and the hyo- and hypo-stemals unite suturally with the fourth, fifth, and sixth marginal plates, forming the side-walls of the bony chamber. The junction between the hyo- and hypo-stemals admits of some yielding movement. The iliac bones, 62, dbut against the pleurmophyses of the tenth, eleventh, and twelfth vertebrae, counting from the first dorsal vertebra. These three vertebrae form the sacrum : their pleurapophyses are uuanchylosed, converge, and unite at their distal extremities to form the articular surface for the ilium. Beyond these the vertebrae, thirty-five in number, are free, with short, straight, and thick pleurapophyses, articulated to the sides of the anterior expanded portions of the centrums. They diminish to mere tubercles in the first caudal vertebra, and disappear in the remainder. The neiu'al arches of the caudal vertebrae are flat above, and without spines. The strong columnar scapula, 51, is attached by ligament to the first costal plate, and, retaining its primitive rib-like form, it descends almost vertically to the shoulder-joint, of which it forms, in common with the coracoid, 52, the glenoid cavity. A strong subcyHndrical process or continuation of the scapula, representing the acromion, bends inwards to meet its fellow at the middle Hne. The coracoid continues distinct from the scapula, expands, and becomes flattened at its median extremity, which does not meet its fellow or articulate with the sternum. The iliac bones, 62, are vertical and columnar, Kke the scapula, but are shorter and more compressed : they articulate, but do not coalesce, with the pubis, 64, and ischium, 63. The acetabulum is formed by coutiguousparts of all the three bones. The pubis arches inwards, and expands to join its fellow at the median symphisis and the ischium posteriorly. It sends outwards and downwards a long thick obtuse process from its anterior margin. The ischia, in Hke manner, expand where they unite together to prolong the symphysis backwards. In the skuU the parietal crista is continued into the occipital one without being extended over the temporal fossae, as in the turtle ; the fascia covering the muscular masses in these fossae undergoing no ossification. The bony hoop for the membrana tympani is incomplete behind, and the columelliform ^apes passes through a notch instead of A foramen to attain the tympanic membrane. The mastoid is excavated to form a tympanic air-ceU. In the Australian long-necked terrapene {hydraspis Imgicollis) the head is much depressed^ the mastoids are excavated by large tympanic cells, and prolonged backwards : the frontal is produced forwards as far as the anterior nostril, where it terminates in a point between the two nasals, which are here distinct from the prefrontals. The margins of the upper and lower jaws are trenchant : the hypapophysis of the atlas has the form of a diminutive wedge-bone, forming as usual the lower part of the articular cup for the occipital condyle : the rest of the body of the atlas, or " odontoid," has coalesced with its proper neural arch, which developes two transverse and two long posterior oblique processes, as in the chelys. 218 LIMBS OP THE TORTOISE AND TURTLE. In the tnio or land tortoises the temporal depressions are exposed, as in the box- tortoises and fresh-water terrapenes ; the head is proportionally small, and can be with- drawn beneath the protective roof of the carapace. The skull is rounder and less depressed than in the teiTapenes : the frontals enter into the formation of the orbital border. The tympanic hoop is notched behind, but the columelliform stapes passes throagh a small foramen. The palatine processes of the maxillaries are on a plane much below that of the continuation of the basis cranii, formed by the vomer ajid palatines. In most of the chelouia the nasal bone is connate with the prefrontal j and, in all, the tympanic pedicle is firmly wedged between the broad appendage of the max- illary arch, formed by the malar, 26, and squamosal, 27, in front, and the mastoid, 8, behind. The broad-headed terrapene {podocnemys expansa) differs from other fresh- water tortoises, and approaches the marine tortoises (turtles), by the vaulted bony roof arching over the temporal depressions. This roof is chiefly formed by the paiietals, but differs from that in the turtles in being completed laterally by a larger proportion of the squamosal than of the postfrontal, which does not exceed its relative size in other ter- rapenes. The present species fui-ther differs from the marine turtles in the non- ossifleation of the vomer and the consequent al)?race of a septum in the posterior nostrils ; in the greater breadth of the pterygoids, which send out a compressed rounded process into the temporal depressions : the orbits also are much smaller, and are bounded behind by orbital processes of the postfrontal and jnalar bones: the mastoids and paroccipitals are more produced backwards, and the entire skuU is more depressed than in the turtles. , The ordinary position of the scapular extremity is a state of extreme pronation, as shown in Fig. 20, with the olecranon, or top of 54, thrown forwards and outwards, and the radial side of the hand, or thmnb, i, directed to the ground. The humerus, S3, is strongly bent in a sigmoid form, with the anconal surface convex and directed upwards and outwards : the two tuberosities at the proximal end are much developed and bent towards the palmar aspect, bounding a deep and wide groove : that which answers to the external tuberosity is the smallest, and by the rotation of the humerus it becomes the most internal in position. The proximal row of the carpus consists of four bones — viz., a large scaphoidcs, a small lunare, wedged into the interspace of the radius and ulna, u large cuneiforme, and a small pisiforme. The sepond row consists of five distinct bones, corresponding with the five digits ; those supporting the fourth and fifth answering to the os imciforme, the remaining three to the trapezium trape- zoides, and magnum. The first and fifth of the digits have each one metacarpal and two phalanges ; the rest, ii, Hi, iv, haye each a metacarpal and three phalanges. A sesamoid bone is placed beneath the metacarpo-phalangeal joint of the three middle digits. In the pelvic extremity, the femur, 65, is sigmoidaUy bent, but in a less degree than the humerus, and is a shorter bone. The patella is ligamentous : the synovial jouit between it and the femur is distinct from the proper capsule of the knee-joint-, s* the fibula, 66, is longer and more slender than the tibia, 66 ; a small " fabeUa" ia articulated to its upper end. The proximal row of the tarsus consists of two bones astragalus and calcaneum, which sometimes become confluent. The distal row consists of five bohes, four of which support the four normal toes, and the fifth, a rudiment of the fifth too without a claw ; the fourth and fifth of the second row of tarsals answer to the OS cuboides of higher animals ; the other three bones to the three ossa cuneiformia. The astragalar part of the single proximal bone would seem to include the scaphoid as well as the calcaneum. SKBIBTOK OP BIRDS. 219 In the marine chelonia the digits of both limbs are elongated, flattened, and united by a web ; the hands and feet having the form of fins. In aU the chelonia the long bones of the limbs are solid, without medullaiy cavities. The Skeleton of Biids.— From the massive frame of the cold-blooded, heavy, and proverbially slow tortoise, to the light, hot-blooded, flying bird, the transition seems to be abrupt, and the discrepancy between creatures so differently endowed extreme ; nevertheless, at the confines of the feathered class, we find some aquatic species, such as the penguin, incapable of fifght, having the wings modified to act as fins, and much resembling those of the turtle ; with the bones solid, and the feathers jr-scmbliag scales. AU birds, like tortoises, lay eggs, are devoid of teeth, and have their jaws sheathed witli horn, and forming a biU. or beak. Most birds, however, enjoy the faculty of flight. If the student of comparative osteology wiU procure the skuU of a rook, a hawk, a swan, or a sea-gull, and vertically bisect it, he will have a ready instance illustrative of some of the characteristics of the osteology of the feathered class. Such a section will show the ivory ^Uke whiteness and compactness of the osseous Mssue, and the loose open cajiceUous structure of the bones. He will see that air is admitted into these can- ceUi partly from the nasal passages, and partly from the tympanic cavity which receives it from the eustachian tube ; from the latter source, the proper bones of the cranium receive their air. Some of the characteristic features in the composition of the skuU of birds may also be noticed : as, for example, the obliteration of all the ordinary sutures of the cranium, except those which unite the tympanic bone, 28, to the mastoid, 8 ; and that which unites the pterygoid, 23, to the basisphenoid, 5 ; which sutures are speedily obliterated in the human subject. The premaxUlary is confluent with the nasal and with the maxOlary ; the nasal being confluent with the frontal and the maxillary with the jugal. The jugal and squamosal are also confluent, and form a long zygo- matic style in aU birds, connected at the hinder extremity by a moveable glenoid joint to the outer and lower part of the tympanic. The pterygoid articulates, in like manner, with the inner and lower part of the tympanic, the movements of which are thus communicated to the upper mandible, so far as the junction of the nasal with the frontal admits of such independent motion. The upper jaw, or mandible, which includes the vomer and nasals with the maxillary arch and appendages, is moveable in a bird through the junction of the nasals and nasal branch of the premaxillary with the frontal, by means of a moveable articulation, or by elastic plates. If the student wiU next separate one of the vertebrae of the trunk from the rest, and cut out that portion of the long and broad breast-bone to which its pair of ribs are attached, he wiE have a segment of the skeleton, answering to that figured in Fig. 5, p. 169. The cut surfaces wiU demonstrate the light ceUulosity of the divided bones. The following letters indicate the elements of such modified vertebrae of the thorax : y, cen- trum, with its hypapophysis ; p, parapophysis ; d, diapophysis ; n, neural arch and rudimental spine ; pi, pleurapophysis ; h, haemapophysis ; hs, hsemal spine. The ten- dency of individual elements and bones to coalesce in birds has already been illustrated in the cranium ; it is shown, in most birds of flight, not orfy by the confluence of the centrum with the neural arch, but by that of several consecutive centrums and arjhcs into a single bone, in the ample chest. In like manner the haemal spines, which con- tinue distinct in many vertebrata, have here coalesced into a single bone, which articu- 220 SKELETOlf OF THE SWAN. lates on each side with the hBemapophyses of several vertebrae, are also much developed in breadth, and send down, from the middle of their under suri face, a longitudinal crest or keel. This modification relates to the extension of the surface for the origin of the great muscles of flight, and renders the " sternum," as the coalesced series of hasmal spines is called) one of the most chaiacteristic parts of the skeleton of the bird. Ossification extends from the neural arches into the tendons of the vertebral muscles, and such bone-ten* dons, both here and in other parts of the body, as the legst are also characteristic of birds. The scapula (Fig. 24), 51, is long and slender, as in the chelonia, but is more compressed and sabre-shaped. The coracoid, 62, as a general rule, is a distinct bone, move- ably articulated to the scapula at one end These coalesced spines Fig. 24.— 8KEI.ET0K OF THE 8WAN (Cyi/nus fcrui). and to the stemmn at the other. Its broad sternal end here articulates by a kind of gomphosis with a deep groove on the fore part of the sternum. The clavicle (ii.), SS, SKELETON OP THE SWAJT, 221 a^tipulates with the coxacoid ahove, but is confluent with its fello-w and with the keel of the sternum below. The iliac bones, 62, are remarkable for their length, and for the number of the vertebrae, or the great extent of the confluent spinal column, to which they are anchylosed, They reach in the awan, and in most other birds, from the tail forwards to the Tertebr^ with moveable ribs. Thus the artificial characters of a " lumbar vertebra" are wanting, The pubis and ischium on eaph side have coaleseed with the Uium to form the lower boundary of the widely-perforated acetabulum. The pubis is long and slender, joins the ischium of its own side near i^s lower extremity, but does not join its fellow ; thus the foramen ovale is defined, but there is no symphysis pubis : the absence of this symphysis facilitates the expulsion of the large ovum with its unyielding calcareous shell. The ischium coalesces posterioriy with the ilium, and ooiiverta the ischiadic notch into a foramen. The caudal vertebra, Cd, arc few in number, with broad transverse processes formed by confluent pleurapophyses, the limits of which may still be traced. A hsemapophysis is articulated to the lower interspace, between the fourth and fifth caudal, and is anchylosed to the sixth. The humerus of some of the larger birds of flight — e. g., the pelican or adjutant crane— is remarkable for its lightness, as compared with its bulk and seeming solidity; it is, in fact, a mere shell of compact osseous tissue. The orifice admitting air to its large cavity is beneath the great tuberosity at the proximal end. The keel is excavated, not only for the reception of an air-cell, but likewise for a fold of the windpipe, which fold expands with age, and lies horizontally in the sub- stance of the back p^t of the sternum. Small pneumatic foramina are situated at the interior and inner surface of the bone, and perforate the articular surfaces for the sternal ribs. In the skeleton of the wild swan {Gxjgnm ferus) (Pig. 2^), here selected as an iUus- tratiou of the ornithic modification of the vertebrate type, there are not fewer than twenty- eight vertebrse, C S D, between the skull and the sacrum, the last six of which, D D, support moveable ribs : of these the first and second pairs ^e free ; the next four are articu}ate(i to tl^e sternum by bony hasmapophyses ; the last five pairs of ribs are attached to the sacrum and also to the sternum ; but the tenth, or last rib on the left side, is very rudimentary, being only about one inch in length. There are eight caudal vertebrae, Cd. The trachea or windpipe penetrates the sternum, ^^ bends and winds in the interior of the bane before returning to enter the chest." The apex of the furculum, 58, bends upwards, and forms a hoop over the windpipe as it enters into the keel of the breast-bone. The furculum, sometimes called " merrythought," consists of the two clavicles confluent at their lower free ends. If a portion of the one side of the sternum be removed, the tortuous trachea which it incloses wiU be exposed. To the great length and peculiar course of the windpipe in this species is to be attributed its remarkably loud and harsh voice ; whence the name hooper, or whistling swan, has been derived ; and is applied in contradistinction to the domestic or mute swan, in which, as in most other birds, the trachea proceeds at once to the lungs, without entering the sternum. In the female of the wild species, the course of the trachea is much more limited than in the male, seldom penetra,ting the sternum to a greater extent than from three to four inches. The breadth of the sternum, and the strong ridge or keel that descends from the mid-line of its under surface, relate to the increased extent of surface requii-ed for the attachment of the "pectoral" muscles, which are the active organs of flight. In the land-birds devoid of the power of flight, such as the ostrich and aptcryx, the keel ia 222 SKELETON OF THE DUCK TEIBE. wanting and the sternum is short. Its various proportions, processes, notches, and per- forations render it a very characteristic bone in birds. In no order, founded upon modifications of the feet, is the sternum more diversified in character than in the palmipedes or web-footed order ; for in none are the powers of flight enjoyed in such dififerent degrees, or exercised in such various ways, from the frigate-bird down to the penguins, where the power of flight is abrogated, and the rudi- mental wings used as fins. In the goose and duck tribes, as well as the swans (anaeres, Linn.), the sternum is long and broad, and presents two moderately wide and deep hind notches ; the costal processes are usually subquadrate ; the eoracoid grooves are continued into one another at the median line ; the costal tract forms about half of the lateral margin in the ducks and geese, and two-thirds or more in the swans ; the interpectoral ridge extends from the prominent part of the eoracoid margin backwards, nearly parallel to the lateral margin, to the inner side of the lateral grooves ; the back part of the sternum between the grooves is quadrate, with the angles slightly produced in most ; there is a short manubrial process below the eoracoid groove. The form of the sternum, its long keel, and the backward production of the long and slender ribs, give a boat- like figure to the trunk of these swimming-birds which is well adapted to their favourite medium and mode of locomotion. The bones of the wing or anterior extremity do not present that extraordinary development which might be expected from the powers of the member of which they form the basis. The great expanse of the wing is gained at the expense of the epidermoid system (quills and feathers, like hairs and scales, are thick- ened epiderm), and is not exclusively produced by folds of the skin requiring elongated bones to support them, as in the flying-fish, fiying-lizards, and bats. The wing-bones of birds are, however, both in their forms and modes of articulation, highly eharac- teristio of the powers and applications of the muscular apparatus requisite for the due actions of flight. The bones of the shoulder consist on each side of a scapula, 51, a eoracoid, 52, and a clavicle, 68, the clavicles being, as a general rule in birds, confluent at their median ends, and so forming a single bone called " furculum" or " os furcatorium •" this further modification of the haemal arch in birds, repeating that of the pubis and lower jaw in some other animals, having occasioned an additional specific term in orni- thotomy. The scapula, 51, is a long, narrow, flat sabre-shaped plate, expanded at the humeral end, where it forms' externally part of the joint for the arm-bone called " glenoid cavity," and extended backwards nearly parallel with the vertebrse, as far as the flium, 62, in the swan, and reaching to the last rib in the swift ; but it is much shorter in the birds incapable of flight. The eoracoid is the strongest of the bones of the scapular arch : it forms the anterior half of the glenoid cavity, extends above this part to abut upon the furculum, and is continued dowiiwards below the joint, expanding to be fijted in the transverse groove at the fore part of the sternum ; it thus forms the chief support of the wing, and the main point of resistance during its downward stroke. In the hawks and other birds of prey, and in the crows and most passerine birds, a small bone (os humcro-capsulare) extends between the scapula and eoracoid along the upper part of the glenoid cavity ; this is absent in the swan and other swim- mers, as well as in the gaUiuaceous and wading birds. The humerus, 53, is usually a long and slender bone, but is not always developed in length in proportion to the powers of flight ; for, although it is shortest in the struthious birds and penguins it is also very short, but much thicker and stronger in the swift and hummin^'-birds. The head of the humerus is transversely oblong and convex ; it is further enlarged by two BONES OF THE WING. 223 lateral crests ; of these the superior is the longest, and is bent outwards ; the inferior is thickened and incurved, and beneath it is situatett the orifice by which the air penetrates the cavity of the bone. The articular surface at the opposite or " distal" end is divided into two parts, one internal, for the ulna, of a hemispheric form, the other also convex, but more elongated and oblique, extending some way upon the anterior surface of the humerus. The extremity of a long bone of a limb which is next the trunk is called the "proximal" one ; the extremity farthest from the trunk the "distal" one: they are not always "upper" and "lower." The ulna, 55, glides upon the inner hemi- spheric tubercle, upon the trochlear canal, and on the back part of the outer convexity. A ligament, extending from the outer part of the head of the radius to the outer pait of the olecranon, above the posterior margin of the outer division of the articular surface of the ulna, plays upon the back part of the radial convexity of the humerus, and com- pletes the cavity receiving it. The ulna is always stronger than the radius ; but both are long, slender, and nearly straight bones, so articulated together as to admit of scarcely any rotation which adds to the resisting power of the wing in the action of flight. The upper part of the ulna, or " olecranbn, " is short. In the tendon attached to it a separate ossicle is developed in the swift, and two such bones in the pen- guin. The ulna is often impressed by the insertions of the great quUl-feathers of the wing. The bones of the hand are very long and narrow, with the exception of the two distinct or unanchylosed carpal bones ; these are so wedged in between the antibra- chium, 54, 55, and the metacarpus, 57, as to limit the motions of the hand to abduction and adduction, or those necessary for folding up and spi'eading out the wing. The hand is thuWfixed in a state of pronation ; aU power of flexion, extension, and rotation is removed from the wrist joint ; so that the wing strikes firmly, and with the full force of the ^depressed muscles, upon the resisting air. The part of the hand numbered 57 in Fig. 2i includes the metacarpal bones of the digits answering to the second, third, and fourth of the pentadactyle members, which are confiuent at their proximal ends with each other, and with the " os magnum," one of the carpal bones, now forming the convex base of the middle metacarpal. This metacarpal and that answering to the " fourth" digit are of equal length, and are also confluent at their distal ends ; but the middle or "third" metacarpal is much the strongest. That answering to the " second" digit, », is very short, and like a mere process from the third ; it supports two short pha- langes in the swan. The third metacarpal supports three phalanges, iii, the fourth a single phalanx, iv. All these are wrapped up in a sheath of integument, and are strongly bound together ; so that the wing loses nothing of its power, whilst so much of the typical structure of the member is retained, that every bone can be referred to its corresponding bone in the most completely developed hand. In ornithology the large quill-feathers that are attached to the ulnar side of the hand are termed " primarise," or primary feathers; those that are attached to the fore- arm are the " secundaxiae," or secondaries, and " tectrices," or wing-coverts ; those which lie over the humerus are called " soapularise," or scapularies ; and those which are attached to the short outer digit, «, erroneously called the "thumb," are the " spuriae," or bastard feathers. The bones of the leg do not present the same number of segments as those of the wing, that corresponding with the carpus being wholly blended with the one that succeeds. The pelvic bones offer this contrast with those of the shoulder, that they are always anchylosed on either side into one piece, " os innominatum" and not at the median 324 PELVIS AND BONES OF THE LEG OF BIRDS. ^— __ — __ ■ — ^ line, whilst this is fte only place where the elements of the scapular apparatus are united by bone. In the young bii-d the os innominatum is composed of three bones. The ilium, 62, is flattened, elongated, usually anchylosed to a very long sacrum : it forms the upper half of the joint for the thigh-bone, called " cotyloid cavity." The pubis, 64, is very long and slender : it does not meet its fellow at the middle line in any bird save the ostrich, but is directed backwards, with its free extremity bent down- wards. The pelvis of the ostrich is so vast, that the pubic junction completing it does not impede the exit of the egg ; in other birds the open pelvis facilitates the passage of that large and brittle generative product. The ischium, 63, is a simple elongated bone, extending from the cotyloid cavity backwarcis, parallel with the iKum ; it sometimes coalesces, as in the swan, with both the iHum and pubjs at its distal end. The cotyloid cavity is incomplete behind, and is closed there by ligament. The femur, 65, is a short, cylindrical, almost straight bone ; the head is a gmaU hemisphere, presenting at its upper part a depression for the "round ligament." The single large " trochanter" generally rises above the articular eminence, and is continuous with the outer side of the shaft. The orifice for the admission of air is situated in the depres- sion between the trochanter and head. The distal end presents two condyles, the inner pne for the inner condyloid cavity of the tibia ; the oute^ one for the outer cavity of the tibia and for the fibula ; the outer condyle is produced into a semicircular ridge, which passes between the tibia and fibula : this ridge puts the oiiter elastic ligament on the stretch, when the fibula is passing over the condyle, and the fibula is pulled into a groove at the back of the condyle, with a jerk, when in extreme flexion ; this spring- joint is weE exemplified in both the swan and water-hen. The proximal end of the tibia is divided into the two shallow condyloid levities above noticed : two ridges are extended from its upper and anterior sui-face : the strong^ of these is the " procnemial" ridge, and is slightly bent outwards : the shorter one on the outside of this is the " ectoonemial" ridge ; th?y are usually united above by a transverse ridge, called " epicnemial" ridge ; this is developed into a long process in the divers, grebes, and guillemots : a fibular riijge projects slightly from the upper third of the tibia for junction with the fibula. The distal end of the tibia forma a transverse pulley or trochlea, with the anterior borders produced. Above the fore part of the trochlea is a deep depression, and in many birds an osseous bridge extends across it. The third segment of the leg, 69, is a compound hone, consisting originally of one proximal piece, short and bybad, presenting two articular concavities to the two thick and round borders of the tibial trochlea, of three metatarsals which coalesce with each other and with the above tarsal piece, and of one or more bony processes which are ossified from the back part of the proximjU piece, or from the proximal ends of the metatarsals, and which, from their relations to the extensor tendons, are called " cal- caneal" processes. In most birds a small rudimeutal metatarsal, supporting the inner.! most toe or " hallux," i, is articultited by ligament with the innermost of the coalesced metatarsals, and is properly included in the same segment of the Kmb. The three principal metatarsals are interlocked together before they become anchylosed the middle one being wedged into the back part of the interspace of the two lateral ones above, and into the fore part below, passing obliq^uely between them. The period at which these several constituents of the " tarso-metataise" coalesce is shorter in the birds that can fly than in those that cannot ; and the extent of the ooalesoenoe is least in the penguins, in which the true nature of the compound bone is best seen STKUCTURE OF THE FOOT IN BIRDS. 225 The modifications of the tarso-metatarse are chiefly manifested in its relative length and thickness, in the relative length of the three metatarsals, and in the number and complexity of the calcaneal processes. The inner of the tsvo cavities for the condyles at the proximal end of the bone is the " eutocondyloid" cavity or surface, the outer one the "ectocondyloid" surface; they are separated by an "intercondyloid" tract, from the fore part of which there usually rises an intercondyloid tuberosity. The cntocondyloid cavity is usually the largest and deepest : it is so in the raven, in which the base of the intercondyloid tubercle extends over the whole of the intercondyloid space. There are three calcaneal processes : one, called the " entocalcaneal," projects from below the entbcondyloid cavity, and from the back part of the upper end of the entometatarse ; a second, called the " meso- calcaneal," from the intercondyloid tract and the mesometatarse, and the third called " ectocalcaneal," from behind the ectocondyloid cavity and the ectometatarse. These three processes are united together by two transverse plates circumscribing four canals, two smaller canals being further carried between the ento- and meso-calcaneal processes. The primitive interosseous spaces are indicated by two small foramina at the upper and back part of the shaft, which converge as they pass foi-ward, and terminate by a single foramen at the fourth part of the anterior concavity. A similar minute canal is retained between the outer and middle metatarsals, near their distal ends ; each metatarsal then becomes distinct, and developes a convex condyle for the proximal phalanx. The middle one is the largest, and extends a little lower than the other two ; it is also impressed by a median groove ; the more compressed lateral condyles are simply convex, and are of equal length. A rough surface, a little way above the inner condyle, indicates the place of attachment of the small metatarsal of the hallux. In the swan and other anserine birds the calcaneal iwominence presents four longi- tudinal ridges, divided by three open grooves, the innermost ridge being the largest ; the shaft is subquadrate, with the angles rounded, and none of the surfaces are chan- nelled. The inner condyle scarcely extends before the base of the middle one ; the canal perforating the outer intercondyloid space is bounded below by two small bars passing from the middle to the outer condyle, and which bars define the groove for the adductor muscle of the outer toe. The tarso-metatarse of the diver [colymbus) is remarkably modified by its extreme lateral compression. The ento- and ecto-calcanca are prominent, oblong, subquadrate plates, inclining towards each other, hut not quite circumscribing a wide intermediate space. The broad outer and inner surfaces of the shaft are nearly flat ; the narrow fore and back surfaces are channelled ; the anterior groove leads to the wide canal, per- forating obliquely the shaft above the outer intercondyloid space, from which a narrower canal conducts to that interspace. The middle and outer trochlese are nearly equally developed ; the inner one stops short at the base of the middle one. ' The number of toes varies in different birds ; if the spur of the cock be regarded as a rudimental toe (which is not, however, my view of it), it may be held to have five toes, whUe in the ostrich the toes are reduced to two. Birds, moreover, are the only class of animals in which the toes, whatever be their number or relative size, always differ from one another in the number of their joints or phalanges, yet at the same time present a constancy in that variation. The innermost or back toe, i (Fig. 24), answering, as I believe, to the "hallux," or innermost digit of the pentadactyle foot, has two phalanges ; the second toe, «, has three, the third toe, m, four, and the fourth toe, «', five phalanges ; I believe the toe answer- ORGANIC NATURE.— No. VIII. P 226 MECHANISM OF FLIGHT IN BIRDS. ing to &£ fifth in lizards and other pentadaotyle animals to bo wanting in tho bird's foot, and the spur, sometimes single, sometimes double, as in Favo bicakciratus, to be a superadded weapon to the metatarsc. As the toes in the tridactyle emeu, cassowary, and bustard, have respectively three phalanges, four phalanges, and five phalanges, we recognise them as answering to the second, thii'd, and fourth in other birds ; the toes in the didactyle ostrich have respectively four and five phalanges, and what is here truly suggestive, the outermost, which is much the smallest and shortest toe, has the greater number of joints, viz., five, thus retaining its ornithic type, as the fourth, or outermost,, toe. The entire form of the body, and consequently that of its bony framework,, in a bird, has special reference to the power of flight. The truiik is an oval with the large end forwards. Tho vertebral column of this part is short and almost inflexible, so that the muscles act to great advantage ; the spine of the neck being long and flexible, the centre of gravity is readily changed fcom above the feet — as when standing or walking — to between and beneath the wings during flight ; when suspended in the air the bird's body naturally falls into that position, which throws the centre of gravity beneath the wings. The axis of motion being situated in a different place in the line of the body when walking from that which is used when flying, the discrepancy requires to be compensated by some means in all birds, in order to enable them to perform flight with ease. Kaptorial bii'ds take a horizontal position when suspended in the air, and the compensating power consists in thcu' taking a more or less erect position when at rest. Another class, including the woodpeckers, wagtails, &c., take an oblique position in the air ; with these the compensating power consists in their cleaving and passing through the air at an angle coincident with the position of the body, and performing flight by a aeries of curves or saltations. Natatorial birds sometimes need very extended flight ; they take a very obliq^ie position in the air, stretch out their legs behind and their neck in front ; thoy have the ribs gi'catly lengthened, the integuments of the abdomen are long and flexible, which enables them greatly to enlarge the abdominal portion of their bodies by inflating it with air ; this causes a ■decrease in the specific gravity of that part, and raises it to a horizontal position ; the compensating power consists in the posterior half of the body becoming specifically lighter, while the specific gravity of the anterior half remains unaltered. When they alight they drop the legs, throw back the trunk by bending the knee-joint, and bring the head over the trunk by a graceful sigmoid curve of the long neck, as in Fig. 24. The act of swimming is rendered easy by the specific gravity of the body, by the boat-lifcB shape of the trunk, and by the conversion of the hinder extremities into oars, in consequence of the membranes uniting the toes together. The effect of these web-feet in water is further assisted by the toes having their membranes lying close together when carried forwards; whilst, on the contrary, they are expanded in striking backwards. The oar-like action of the legs is stiU farther favoured by their backward position, — an arrangement, however, which is unfaYOurable for walking. Borelli was the first who, by compairdsou of the anatomical peculiarities of the human frame and tho structure of birds, demonstrated, to a certain extent, the impossi- bility of the realization of the cherished project of flying by man. He arrived at this conclusion from a comparison of the form and strength of the muscles of the wings of birds with the corresponding muaclea of the human body. Pzincipal Foims of the Skeleton in the Class Blammalia. In the class Mammalia, which includes the hairy quadrupeds with the naked apodal whales and VAKIOUS FOEMS 01' LIMBS IN MAMMALS. 22? biped man, the form of the animal is modified for a great diversity of kinds and spheres of loeomotion. Some lire exolnsiyely in the ocean, and cleave the liquid dement under the form and with the locomotive powers of fishes; some frequent the &esh waters; some pass a subterraneous existence, and work their way through . the solid earth ; some mount aloft; to seek and seize their prey in the air ; some pass their lives in trees ; most, however, dwell on the earth, with various powers of walking, running, and leaping. Lastly, man is modified to sustain his frame erect on the hinder, now become in him the lower, limbs. In the Mammsliaoi class, accordingly, we find the limbs progxessively endowed with more varied and complicated powers. They retain in the Ceiacea (whale and porpoise tribe) their primitive form of flattened fins ; in the Xfngiilata (hoofed beasts) one or more of the digits acquire the full oomplemoiLt of joints, but have the extremity enve- loped in a dense hoof; in the Ungmculata (quadrupeds with claws) the Umbs, with ampler proportions, have the digits liberated, and anned with claws confined to the upper surface, leaving the under surface of the toes free for the exercise of touch ; in the mote the hand is shortened, thickened, expanded, and converted into a sort of spade ; in the bat the fingers are leng-thened, attenuated, and made outstretchers and supportera of a pair of wings ;• in the Quadrumana (ape and monkey tribes) certain digits are endowed with special ofiices, and by a particular position enabled to oppose the others, so as to seize, retain, and grasp. Lastly, in Man the ofiices of support and locomotion are assigned to a single pair of members ; the anterior, and now the upper, limbs being left free to execute the various purposes of the wUl, and terminated by a hand, which, in the matchless harmony and adjustment of its organization, is made the suitable instrument of a rational being. In contemplating and comparing the skeletons of a scries of mammals, the; most striking modifications are observable in the structure and proportions of the limbs. There are a few osteological characters in which all mammalia agree, and by which they differ from the lower vertehrata; and some have been supposed to be peculiar to them that are not so. The pair of occipital condyles, e. g., developed from the exoccipitals, ai-e a repetition of what we saw in the hatrachia. The flat surfaces of the bodies of the trunk-vertebr£e were a character of many extinct reptiles ; hut these surfaces in mammals are developed on separate epiphysial plates, which coalesce in the course of growth with the rest of the centrum. Moveable ribs, projecting freely (pleurapophyses) in the cervical region, may be found in a few exceptional cases (sloths, monotremes) ; bony sternal ribs (hajmapophyses) exist in most Edentata ; a coraeoid extending, as in birds and lizards, from the scapula to the sternum, with an " epicora- coid," as in lizards, is present in the monotremes (platypus or duck-mole, and echidna or spiny ant-eater, of Australia) ; the cotyloid cavity may be perforated in the same low mammals as in birds ; the digits may have the phalanges in varying number in the same hand, and exceeding three in the same finger, e. g., in the whale tribe. But, the following- osteological characters" are both common and peculiar to the mammalia.. The squamosal^ 27, or second bone of the bar continued backwards from the maxillary areh, is not only expanded as in the chelonia, hut developes the articular surface for tiie mandible, and this surface is- either concave at some part or is flat. Each half or ramus of the mandible- is ossified fr-om a single centre, and consists of one piece; and the condyle is either convex or is flat, never concave. The presphenoid (centrum of the parietal vertebra) is developed distinctly Horn the basisphenoid ; it may become confluent,, but is not connate, therewith. 228 SKELETON OF THE WHALE. One known mammal (the three-toed sloth) has more, and one (the manatee or sea- cow) has less than seven yertehrae of the neck. In the rest of the class these vertehrse, which have the pleurapophyses short and usually anchylosed, are seven in numher. - Skeleton in the Cetacea or Whale Tzibe In the skeleton of the whale (Fig. 25), which to outward appear- ^'^' ^^' ance seems to have aa little neck as a fish, there are as many cervical ver- tehrse as in the long-necked girafife : this is a very striking instance of adherence to type within the limits of a class : the adaptation to form and function is effected by a change of proportion in the bones ; the cervical vertebrse in the whale are flattened 'fi *f?5!!a*^'^-^^ISa' ^'^a^.^T/ fi^oni before backwards into broad thin J\ "^^^^W^^^^ ^^...Siay plates ; in the giraffe (Fig. 30) they ^ ^""^S^^T are produced into long subcyUudrical roKK-sHOKTENED VIEW OP THE sKELETou 01' A wBAiE boucs. lu thc whales thc movements (Baltenoptera boops), showing its eelattve size to n ., , , MAK. -r -^ " ot these vertebrae upon one another are abrogated, and in the grampus and porpoise the seven vertebra are blended together into a single bone ; they thus give a firm and unyielding support to the large head, which has to overcome the resistance of the water when the rapid swimmer is cleaving its course through that element. The dorsal vertebras are characterized in aU mammalia by the sudden increase in the length and size of the ribs, which, in a certain number of these vertebrae, including the first, are joined to a breast-bone by a commonly cartilaginous, rarely osseous, part. The first rib is remarkable for its great breadth in the whale ; this and a few following ribs are joined to a short and broad and often perforated sternum (Fig. 25), No. 60 ; the remaining ribs are free, or, as they would be called in Human Anatomy, "false." They are articulated to the ends of diapophyses, which progressively increase in length to the last of the dorsal series. Then follow vertebrfe without ribs, answering to those called " lumbar." The whole hinder part of the trunk of whales being needed to effect the strokes by which they are propeEed, its vertebras are as free from anchylosis as in fishes ; there is consequently no " sacrum," and the caudal vertebrae are counted from the first of those that have " chevron bones" articulated to their under part. This special name is oiven to the vertebral elements called "haemapomophyses" (see Fig. 26, /<), which are ai-ticulated in cetacea as in crocodilla, directly to the under surface of the centrum, and, coalescing at their opposite ends, develope thence a "hasmal spine," and form a "hjemal" canal analogous to, but not homologous with, that in fishes (compare No. V, h, with No. I p in Cut 10, p. 182). The caudal vertebrae of whales further differ fi'om those in fishes in retaining the transverse processes, and in becoming flattened from above downwards, without coalescing. These modifications relate to the support of a caudal fin which is extended horizontally instead of vertically. Whales and porpoises progress by bounding movements or undulations in a vortical plane, and their necessity of coming to the surface to inhale the air directly, as warm- blooded mammals, calls for a modification in the form of the main swimming instrument such as may best adapt it to effect an easy and rapid ascent of the head. The course of the whale is stopped and modified by the action of the pectoral Hmbs, SKELETON OF THE DDGONG. 229 which are the same parts as those in fishes, hut constructed more after the higher ver- tebrate type. The digital rays do not exceed five in numher ; hut they consist of many flattened phalanges, and are enveloped in a common sheath of integument. A radius, 55, and an ulna, 64 (Fig. 25), support the carpal series ; hut, instead of heing directly articulated to the scapular arch, they are suspended to a humerus, 53 : this is a short, thick hone, with a rounded head. The scapula, 51, is detached from the occiput, has a short, stunted, coracoid anchylosed to it, and is thus freely suspended in the flesh ; it developes an acromial process : the ulna, 54, is produced upwards into an olecranon. "With all those marks, however, of adhesion to the mammalian type of fore-arm, the out- ward aspect of the limb is as simple as is that of the fish's fin ; it moves, as by one joint, upon the trunk, and is restricted to the functions of a pectoral fin. In the huge skuU of the whale the broad vertical occiput may he noticed, by which the head is connected, through the medium of a short consolidated neck, with the trunk; the whole cranium seems to have been compressed above, from before backwards, so that the small nasal bones, 15, articulating with the short and very broad frontals, form the highest part of the skull. The long maxiUaries, 21, and premaxillaries, 22, extend backwards and upwards, to articulate with the nasals, and complete with them the bony entry to the air-passages, situated so favourably at the summit of the cranium. The nostrils, formed by the soft parts guarding that entry, are called " blow-holes ;" they are double in the whales— single in the smaller cetacea. In the whales the " baleen" or " whalebone" plates are attached to the palatal surface of the maxillary and pre- maxiUary bones ; the expanded toothless mandible supports an enormous under lip, which covers the whalebone plates when the mouth is shut. The skeleton of the great finner whale {Balcenoptera hoops), from which the foreshortened view (Cut 25) is taken, was ninety-six feet in length ; the relative dimensions of man is given by the outlines of the skeleton at its side. No known extinct animal of any class equalled this living Leviathan in bulk. There are a few whale-like mammals, equally devoid of rudiments of hinder Hmbs, which obtain their sustenance from sea-weeds or sea-side herbage. They have teeth Fig. 26. SKELETON OF THE DUGONG {Salicore Australis)- adapted for bruising such substances, and the movements of the head in grazing require the cervical vertebras to be unanchylosed ; these are, however, short, and in the manatee but six in number. In the dugong (Fig. 26), one of these herbivorous sea-mammals frequenting the Malayan and Australian shores, the upper and lower 230 SKELETON OF THE WALKUS. jaws are singularly bent down, and tlio nppor jaw is aimed with a pair of short tusks. The bones of all these cetacoa arc singularly massive and compact. Three or four of the anterior thoracic ribs are joined to a sternum — the rest are free. One of the vertebras intervening between the costal and caudal series has connected with it a simple pelvic ai-ch, in which the ilium and ischium may be recognised, and a still more rudimental condition of such arch is suspended in the ingiiinal muscles of the true cetacea. Most of the caudal Tcrtobrcc (Fig. 26), ed, cf the manatee and dugong, have long diapophyses, and htemal arches (Fig. 2fi), /*. The terminal vertebra; are flattened horizontally. The lacteal organs of the dugong are placed on the breast, and the pectoral fins, in the female at least, are applied to clasp the young ; and the animal ao observed, with its own head and that of its young above water, has given rise to the fable of the siren and mermaid. The bones and joints of the pectoral fin are accordingly better developed than in the ordinaiT whales. The first row of carpal bones, 56, consists of two — one artictilated to the radius, 65, the other to the ulna, 54, and fifth digit, 57, ■'', and both to the single bone representing the second row. The first digit, i, consists of a short metacarpal ; the mctacai'pals of the others support each three phalanges. Skeleton of tSie Seal. — In tike seal tribe {Fhodda) another and wcU-marked stage is gained in the development of the ten-estrial instruments of locomotion. Hind limbs are now added — the marine mammal has become a quadruped. The sphere of life of the seals is near the shores ; they often come on land ; they sleep and bring forth among the rocks and littoral caves : hence the necessity for a better development of the pecWral limbs, although these, Hkc the pelvic ones, still retain the general form of £ns. The fish-hxmting seals make more use of the head in independent movements of sudden SKi-iLKTON or THE WALRUS [Tficlicciis roswarus). extension, retraction, and quick turns to the right and left, than do the ectaeea of like diet ; and the wah-us (Fig. 27) works the head, as the place of attachment of its long, Virtieal, dowc-gi'owing tusks, in various movements required in clambering over floes and berg.5 of ies. Accordingly, in the seal tribe we find the seven neck-vertebra» (ib.) c, longer, and with more finished and free-playing joints than in the whales anddugongs. The sigmoid cua-vc, in which they can be thrown during rotraetion of the head, exceeds SKELETONS OF THE SEAL AND WALKUS. 231 that in most other manunals, and almost reminds one of the extent of flexion of this part of the spine in birds. In the -walrus, the skeleton of which is here selected to exemplify the phooal modi- fication of the mammalian skeleton, the vertebral formula is : — 7 cervical, 0, 11 dorsal, D, 5 lumbar, L, 3 sacral, S, and 9 caudal, cd. As, in consequence of the presence of hind-limbs, a sacrum is now established, the characters of the above five kinds of body- vertebrae, as defined in man and other mammals, may here be given : the cervical or neck vertebrce " have perforated transverse processes," the dorsal vertebrso " bear ribs;" the lumbar vertebriE " have imperforate transverse processes and no ribs ;" the sacral vertebras " are anchylosed together ;" the rest are caudal vertebrae whatever their modi- fications. In the above characters, the term '( rib" is given to the vertebral element called " pleui-apophysis," when this is long and moveable ; that element may be, and often is, present, but short and fixed, in both cervical, lumbar, sacral, and caudal verte- brae ; in some mammals, e.g., monotrcmes, the pleui-apophysis may remain unan- chylosed in some of the neck-vertebrEe, but it is short, like a transverse process ; and the so-called " perforated transverse process' ' in all mammals consists of the diapophysis, paia- pqphysis, and pleurapophysis ; the hole being the interval between those parts : in the lum- bar vertebrae the pleurapophysis is short, and confluent or connate with the diapophysis. Keturning to the skeleton of the walrus, wc find that nine pairs of ribs directlyjoin the sternum, which consists of eight bones. The transverse processes of the last cervi- cal are imperforate, consisting of the diapophysis only. The neui'al arches of the middle dorsal vertebrce are without spines and very narrow, leaving wide unprotected intervals of the neural canal. The bones of the neck are modified to allow of great extent and freedom of inflection. The perforated transverse processes of the third to the sixth ccrvicals inclusive are remai'kablo for the distinctness of their constituent parts. Inferior ridges and tuberous processes, called " hypapophyscs," are developed from some dorsal and lumbar vertebrae. These processes indicate the great development of the anterior vertebral muscles, e.g. the "longi colU" and "psoae," and relate to the important share which the vertebrae and muscles of the trunii take in the locomotion of the seal-tribe, especially when on dry land, where they may bo called " gastropods," in respect of their peculiar mode of progression. The wafrua. alone seems to have the power of supporting itself on the fore fins, so as to raise thr belly from the ground. There is no trace of clavicle in any seal. The upper part of the scapula exceeds the lower one in breadth. The spine terminates" by a short and simple acromion. The humerus is short and thick, and is remarkable for the great development of the inner tuberosity and of the deltoid ridge, which is deeply excavated on its outer side. The inner condyle is perforated. The scaphoid and lunar bones are connate. Although the poUox or the first digit exceeds the third, fourth, and fifth in length, it presents its characteristic inferior number of phalanges, by which the front torder of the fin is ren- dered more resisting. The pClvic arch is remarkable for the stunted development of the Hia, and the great length of the ischia and pubes. The femm- is equally peculiar for its shortness and breadth. The tibia and fibula present the more usual proportions, and are anchylosed at their proximal ends. The bones of the foot are strong, long, and are modified to fonn tie basis, of a large aud powerful fin : the middle toe is the shortest, and the rest increase in length to the margins of the foot ; the inner toe has, ndvertheless, but two phalanges, the rest having throe phalanges, whatever then- length ; and this is the typical character, both as to the number of the digits and their joints, in both fore and hind foet of the mamm".lia. 232 SKELETON OF THE HOKSE. In the living walrus and seal the digits of each extremity are not only hound toge- ther by a eommon hroad web of sldn, but those of the hind-limbs are closely connected with the sliort tail : being stretched out backwards, they seem to form with it one great horizontal caudal fin, and they constitute the chief locomotive organ when the animal is swimming rapidly in the open sea. The long bones of seals, hke those of whales, are solid. With regard to the skull in the seal-tribe, it may he remarked that an occipito- sphenoidal bone is formed, as in man, by the coalescence of the basioccipital with the basisphenoid ; the parts of the dura mater or outer membrane of the brain, called " ten- torium," with the posterior part of the " falx," ai-e ossified. The sella turcica is shal- low, but weU defined behind by the overhanging posterior chnoid proce.9ses : the petrosal shows a deep transverse cerebellar fossa, and is perforated by the carotid canal. The frontal forms a small rhiuencephalic fossa, and contributes a very large proportion to the formation of the orbital and olfactory chambers. In Fig. 27, 62 is the iliimi, 63 the ischium, and 64 the pubes, 65 is the femur, or thigh-hone, 66 the tibia, 66' the patella or knee-pan, 67 the fibula, 68 the tarsus, and 69 the metatarsus and phalanges of the hind-foot ; the numbers on the other bones correspond with these in the skeleton of the dugong. Skeletons of Hoofed Quadrupeds— The Horse.— The contrast, as regards Fig. 28. HOUSE {Hquus caballue). the sphere of life and kind of movement between the seal and the horse is very oroat ■ the instruments of locomotion, and indeed the whole frame, need to bo very diflfercnt in an animal that can only shuffle on its bcUy along the ground, and one that can SKELETON OF THE HORSE. 233 traverse the surface of the earth at the rate of four miles in six minutes and a half, as was achieved hy the noted racer " Flying Childers." The modifications in the form and proportions of the locomotive merahers are accordingly extreme. The limhs in the horse arc as remarkahle for their length and slendcmess, as in the seal for their brevity and breadth. Both fore and hind limbs in the horse terminate each in a single hoof ; the trunk is raised high above the ground, and is more remarkable for its depth than breadth, especially at the fore part; the neck is long and arched; the jaws long and slender, being produced so as to facilitate the act of cropping the grass, and leaving so much space between the front teeth, i, and the grinders, m, as permits man to insert the instrument called " bit" into the mouth, whereby he masters and guides his noble and valuable four-footed ally, as the ship is steered by the helm. Were every animal constructed expressly and exclusively for its own peculiar habits of life, and irrespective of any common pattern, it could scarcely be expected, beforehand, that the same bones would be found in the horse as in the seal ; yet a comparison of their skeletons. Cuts 27 and 28, will demonstrate that this is, to a very great degree, the case. The vertebral formula of the horse is : — 7 cervical, C, 19 dorsal, D, 5 lumbar, L, 5 sacral, S, and 17 caudal. Eight pairs of ribs directly join the sternum, 60, which con- sists of seven bones and an ensiform cartilage. The neural arches of the last five cer- vical vertebra; expand above into flattened, subquadrate, horizontal plates of bone, with a rough tubercle in place of a spine : the zygapophyses, z, are unusually large. The perforated transverse process scuds a pleurapophysis, pi, downwards and forwards, and a diapophysis, d, backwards and outwards, in the third to the sixth cervieals inclusive : in the seventh the diapophysial part alone is developed, and is imperforate. The spinous processes suddenly and considerably increase iu length in the first three dorsals, and attain their greatest length in the fifth and sixth, after which they gradually shorten to the thirteenth, and continue of the same length to the last lumbar. The lumbar diapo- physes are long, broad, and in close juxtaposition ; the last presents an articular con- cavity adapted to a corresponding convexity on the fore part of the diapophysis of the first sacral. The scapula, 51, is long and narrow, and according to its length and obli- quity of position the muscles attached to it, which act upon the humerus, operate with more vigour, and to this bone the attention of the buyer should be directed, as indica- tive of one of the good points in a horse. The coracoid is reduced to a mere confluent knob. The spine of the scapula, 51, has no acromion. The humerus, 53, is remarkable for the size and strength of the proximal tuberosities in which the scapular muscles are implanted. The joint between it and the scapula is not fettered by any bony bar con- necting the blade-bone with the breast-bone ; in other words, there is no clavicle. The ulna, represented by its olecranal extremity, 54, is confluent with the radius, 55. The OS magnum in the second series of carpal bones, 56, is remarkable for its great breadth, corresponding to the enormous development of the metacarpal bone of the middle toe, which forms the chief part of the foot. Splint-shaped rudiments of the metacarpals, answering to the second, ii, and fourth, iv, of the pentadaotyle foot, arc articulated respec- tively to the trapezoides and the reduced homologue of the unciforme. The mid-digit, iii, consists of the metacarpal, called " cannon-bone," and of the three phalanges, which have likewise received special names in Veterinary Anatomy, for the same reason as other bones have received them in Human Anatomy. " Phalanges" is the " general" term of these bones, as being indicative of the class to which they belong, and "haem- apophyses" is the " general" term of parts of the inferior arches of the head-segments ; and just as, from the modifications of these hsemapophyses, they have come to be called 234 SKELETON OF THE RHINOCEKOS. '' maxilla," " mandiliula," " ceratoliyal," &c., so the phalanges of the horse's foot are called — the first, " great pastern bone," the second, " small pastern bone," and the third, -which supports the hoof, the " coffin bone ;" a sesamoid ossicle between this and the second is called the " coronary." The ilium, 52, is long, oblique, and narrow, like its homotype, the scapula ; the ischium, 63, is xmusuaUy produced backwards. The extreme points of these two bones show the extent to which the bending muscles and extending muscles of the leg are attached ; and according to the distance of those points from the thigh-bone the angle at which they are therein inserted becomes more favourable for their force ; the longer, therefore, and the more horizontal the pelvis, the better the hind-quai-tcr of the horse, and its qualities for swiftness and maintenance of speed depend much on the "good point" due to the development of this part of the skeleton. The femur, G.5, is characterized by a third trochanter springing from the outer part of the sliaft before the groat trochanter. There is a splint-shaped rudiment of the proximal end of the fibtila, 67, but not any rudiment of the distal end. The tibia,, 66, is the chief bone of the leg. The hecl-bono, " calcaneum," is much produced, and forms what is called the " hock." The astragalus is characterized by the depth and obliquity of the superior trochlea, and by the extensive and undivided anterior surface, which is almost entirely appropriated by the naviculare. The external cuneiforme is the largest of the second series of tarsals, being in proportion to the metatarsal of the large middle digit, Hi, which it mainly supports. The diminished cuboides articulates partly with this, partly with the rudiment of the metatarsal corresponding with that of the fourth toe, ii\ A similar rudiment of the metatarsal of the toe, corresponding with that of the second, //, articulates with a cuneiforme medium — ^herc, however, the inner- most of the second series of tarsal bones. Of aU the other known existing hoofed quadrupeds, it would hardly be anticipated Fig. 29. SKELETON OP THE nniNoCEROS (iJ7t. Hconm). that the rhinoceros presented the nearest affinity to the horse ; one might rather look to the light camel or dromedary ; but a different modification of the entire skeleton may SKELETON OF THE KHINOC'EROS. 233 .be traced in the animals witli toes in even number, as compared with the horse and other odd-toed hoofed quadrupeds. In an extinct kind of horse {Hippopetherium), the two splint-bones are more developed, and each supports three phalanges, the last being provided with a diminutive hoof. In the extinct Falmoiheria the outer and inner digits acquired stronger proportions, and the entire foot was shortened. The fa'ansition from the Palaotheria, by the extinct hornless rhiuoceros [Accrothcrimn), to the existing forms of rhinoceros, is completed. In the skeleton of the rhinoeeres we find resemblances to the hori