Chapter XIV: Part II: Reptilia (2)

_Cynognathus_, Karroo formation of South Africa. _C. crateronotus_ has a skull about 16 inches long, looking like that of a ferocious Carnivore; there are four incisors, huge canines, and nine molars, the latter with serrated edges and anterior and posterior cusps. The wide supratemporal fossa is bordered and closed behind by the broad lateral extension of the parietal, which joins a similar extension of the squamosal bone. The latter is very long, extending to the postfrontal and to a bone which, bordering the orbit posteriorly, is either an upward branch of the jugal, or a postorbital bone; the latter interpretation is made probable by the occurrence of a suture with the jugal in _C. platyceps_. The jugal bone is very long, beginning at the quadrate, running along the squamosal, and forming the lower border of the orbit.

The number of vertebrae is large, there being as many as twenty-nine presacrals, six of which belong to the cervical region. {307}The atlas is fused with the axis; most of the thoracic ribs articulate partly upon the intercentra. The lumbar ribs are very peculiar; they are much expanded horizontally, and overlap each other, forming thereby intercostal foramina. The broad ilium is attached to three or four sacral ribs. The acetabulum is closed. The ventral side of the pelvis shows a broad symphysis and has a pair of obturator-foramina. The scapula is large, directed backwards, and shows a distinct, very Mammalian spine; it is fused with the coracoid and precoracoid.

The occipital condyle of _C. platyceps_ is kidney-shaped, with the concavity directed upwards; in _C. berryi_ it is separated into two distinct knobs, the middle, basioccipital portion being apparently wanting. The mandible possesses a long coronoid process which ascends obliquely into the temporal fossa.

_Aelurosaurus_, _Lycosaurus_, _Galesaurus_, and many others, likewise of the Karroo formation. In the first genus the splenial bones help to form the symphysis of the lower jaw; teeth are also found on the palate, in opposition to _Lycosaurus_. This has a skull 6 inches in length; the dental formula on either side is _i._ 4/3, _c._ 1/1, _m._ 5/5; the molars are slender, conical, and recurved. _Galesaurus_ seems to have been rather small, the low, triangular skull measuring only 2 to 3 inches in length, with four or five sharply pointed incisors, prominent canines and four or five small multicuspid or deeply serrated little molars.

_Endothiodon_, with several species from the Karroo formation, is of uncertain systematic position, only imperfect skulls being known. The animals must have been large and bulky, the skulls being very massive and at least one foot in length. The premaxillaries and the maxillaries are toothless, their alveolar borders forming cutting, prominent edges. The same applies to the very strong lower jaw; but there is a pair of tooth-like stout projections in the upper and lower jaws in the place of canine teeth. True, enamelled, small, apparently conical or low and perhaps blunt teeth occur on either side in one or three longitudinal series upon the palate, and in corresponding positions on the inner sides of the two halves of the lower jaw. It is doubtful if the upper teeth are carried by the palatines or by the broadened inner flanges of the maxillaries. The choanae seem to lie between the pterygoids and the palatines, incompletely roofed in by ventral extensions of the latter towards the middle line.

{308}Direct affinity of _Endothiodon_ (ἐνδοθί, within) with _Placodus_ is unlikely; the same applies to the Dicynodontia, although the restriction of the teeth to the palate seems to point as much to the former genus as do the toothless cutting edges of the jaws to the forms like _Oudenodon_.

Other Theriodont reptiles have been described from the upper Permian of Russia, for instance _Deuterosaurus_ and _Brithopus_, but the determination rests upon insufficient fragments. North America has yielded many strange Theromorphous fossils, some of which may belong to the Theriodont order, while others seem to be intermediate between this and the other orders. _Diadectes_ of Texas, for instance, seems to be a Theriodont creature; while in _Empedias molaris_, with a skull about 8 inches in length, the teeth form an uninterrupted series without distinct canine tusks, and the incisors are distinguished from the molars only by the transversely broadened shape of the latter. Very small teeth are arranged along the median line of the vomer and united palatine bones. In _Clepsydrops_, _Dimetrodon_, and _Naosaurus_ of Texas the teeth are differentiated into incisors, canines, and molars, although not so regularly as in the typical Theriodont forms described above, one or more pairs of teeth being enlarged into canine-like tusks. In the latter two genera the spinous processes of the thoracic vertebrae are enormously elongated, standing up vertically to a height of 2 feet, while the centra of the vertebrae measure only one inch in diameter. In _Naosaurus claviger_ these upright spines carry on either side half a dozen transverse projections. _Stereorhachis_ of the Permian of France is typically Theriodont in the structure of its shoulder-girdle, humerus, and pelvis, but the dentition is composed of 3/3 incisors, no canines, and 6/10 pointed molars.

The following genera have been placed by Seeley in the family Gomphognathidae. _Microgomphodon_, with broader and less prominently multicuspid teeth than those of the typical Theriodonts, seems to lead to _Gomphognathus_, which has the following dentition: _i._ 3/3, _c._ 1/1, _m._ 12/12, with a long diastema between the canines and molars, some of which latter are nearly as broad as they are long, and have comparatively low tubercles on the crowns. The skull is remarkably like that of a Carnivorous Mammal. There are incisive foramina behind the premaxilla. The maxillaries and palatines form a united palatal roof, and behind them open the {309}choanae. The occipital condyle is kidney-shaped. The mandible is most extraordinary, approaching that of the Mammalian, especially the Marsupial type, except that it is still composed of several pieces. The articular facet for the mandible is borne by an outward or lateral projection, while the bulk of the posterior half of the jaw projects inwards like a broad flange, undoubtedly recalling the so-called inner inverted angle of the Marsupial jaw. The coronoid process is large and extends far into the temporal fossa. Nearly the whole skeleton of _Microgomphodon_ is known; the lumbar ribs are broadened and overlap as in _Cynognathus_, and the mandible is typically compound, so that there is no doubt about the affinities of this genus with the Theriodontia. It throws light upon _Gomphognathus_ and the three likewise South African genera _Diademodon_, _Trirachiodon_ and _Tritylodon_, which are all known from imperfect skulls only. Their teeth are restricted to the jaws, the molars have flat, multitubercular crowns and bear an extraordinary resemblance to those of Mammals. Some of the molars of _Tritylodon_ are said even to possess two roots, but this point, absolutely unique in Reptiles, but common in Mammals, is not certain. The few upper incisors of _Tritylodon_ are rather large, chisel-shaped, and extend like those of the Rodent-type back into the maxillaries; canines are absent, leaving a diastema. _Trirachiodon_ has prominent canines, the five upper molars are multitubercular, rather flat, and much broader transversely than in the longitudinal direction. Still, even these creatures, with skulls of the size of that of a small fox, possessed distinct prefrontal and postfrontal bones, and are, at least in this respect, typical Reptiles.

ORDER III. ANOMODONTIA.

The cranium is not roofed in. The pedicle for the suspension of the lower jaw is much elongated, slants slightly forwards, and is composed of the long quadrate, which is laterally overgrown by the squamosal bone. The teeth are restricted to a pair of strong, tusk-like canines, or they are altogether absent. The margins of the upper and especially those of the lower jaw are trenchant, and were possibly furnished with a thick horny armature like those of tortoises.

{310}_Dicynodon_, with many species from the Karroo formation of South Africa, reached formidable dimensions. The thick, curved skull is in size and outline not unlike that of a large lion, hence _D. leoniceps_, _D. tigriceps_, etc. The zygomatic arch is almost mammalian, except that the posterior boundary of the orbit is formed by a distinct postfrontal bone. The nostrils are lateral. The canine tusks (Fig. 54, E, p. 280) are very large. The choanae open behind the rhomboid vomer and between the separated palatine bones, which are posteriorly confluent with the medially united pterygoids. The latter send out flat extensions, along the lateral side of the palatines; these extensions reach the maxillaries and probably represent the ectopterygoids. The occipital condyle is distinctly triple, being equally composed of the basi- and latero-occipital bones.

The three bones of the shoulder-girdle meet at the glenoid fossa; the scapula has the indication of a spine. The pelvis is stout, attached to four or five vertebrae, converting the latter into a very Mammalian-like sacrum, the position of which lies distinctly in front of the acetabulum. The latter is closed, composed by the three pelvic bones. The pubes and ischia are fused together, leaving only a very small obturator-foramen. The limbs are plantigrade and pentadactyle, very stout; the humerus and femur have enormous crests.

_Oudenodon_, of which several species have been described, is so much like _Dicynodon_, except for the complete absence of teeth, that it has been suggested that these skulls belong to females of this genus. This view is strengthened by the fact that tusk-like canines exist, or are absent in some of the species which have been described as _Cistecephalus_, a genus closely allied to _Dicynodon_. The latter, which, like _Oudenodon_ and _Cistecephalus_, occurred in Africa, extended also into India, _D. orientalis_ having been found in the Panchet formation of Bengal, of transitional age between the Permian and Triassic epochs. _Oudenodon rugosus_, on the other hand, has been described from the Ural.

_Gordonia_ and _Geikia_, of the New Red Sandstone of Elgin, are known from their skulls only, but these are so well preserved that there is no doubt about their close relationship to the typical South African Dicynodontia. The skull of _Gordonia_ is about 7 inches long and 4 inches high. The canines (Fig. 54, D, p. 280) are reduced to short, but thick, conical tusks. The most {311}remarkable feature is the very elongated squamoso-jugal arch, which arises moreover from the dorsal end of the long squamoso-quadrate pedicle. The two wide and long temporal fossae are dorsally divided by narrow parietal crests. There is a distinct interparietal bone, and the usual interparietal foramen. The choanae are united and lie within the palatines, which themselves are united; the large lateral palatal foramina are otherwise enclosed by the pterygoids, quadrates, and laterally by the squamoso-jugal arch.

ORDER IV. PLACODONTIA.

These are the latest and last members of the Theromorpha, unfortunately known from skulls only, from the Muschelkalk or Middle Trias of Germany and Russia. The skull of _Placodus gigas_ is about one foot long, rather high and triangular owing to the lateral expansion of the temporal arches, which diverge posteriorly. The squamoso-jugal arch is very broad, and most of the posterior border of the orbit is formed by the large postorbital bone. The maxillary bone seems to extend back to beyond the level of the orbits. The choanae lie behind the premaxillaries. The palatines and pterygoids are fused in the middle line, forming a broad bony palate, which, owing to the broad, posteriorly extended wings of the pterygoids, much resembles that of the crocodiles. The teeth are very remarkable. There are two or three stout, conical, or chisel-like teeth in each premaxillary bone, and three to five broad and flat maxillary teeth; three pairs of huge, broad, and quite flat teeth are crowded together and fill up the whole vomerine and palatine portion of the palate. These crushing teeth indicate that _Placodus_ probably lived upon hard-shelled molluscs, and this would be in conformity with its occurrence in the Muschelkalk, which is a strictly marine deposit and full of shells. Another closely allied genus is _Cyamodus_, one species of which is known from Russia. The teeth are fewer in number and not so large as those of _Placodus_.

{312}CHAPTER IX

CHELONIA–ATHECAE–THECOPHORA

_SUB-CLASS IV.–CHELONIA._

There is no mistaking a tortoise. The shell and the horn-covered toothless jaws separate them from all other four-footed creatures.

They may be described as terrestrial or aquatic, pentadactyle reptiles, with walking limbs or with paddles; ribs with capitular portions only, two sacral vertebrae, humerus with entepicondylar foramen, pubes and ischia forming symphyses, quadrate bones fixed, jaws without teeth, but with cutting horny sheaths. Trunk encased in a bony shell, composed of numerous dorsal and ventral dermal bones, forming a carapace and a plastron, which may or may not be covered with horny shields. Copulatory organ unpaired, cloacal opening more longitudinal than round, never transverse. Oviparous.

It is customary to distinguish the marine, paddle-limbed kinds as _Turtles_, the others as _Land-_ and _Water-tortoises_.

Tortoises occur already in the Trias. They reached their greatest development towards the end of the Mesozoic and in the earlier Tertiary periods. They are now comparatively reduced in the number of families and genera, although they are still represented by about 200 species. The sub-class as a whole is cosmopolitan, but does not occur in the colder regions.

Their origin is quite unknown. Of recent groups only the Crocodilia and the Rhynchocephalia come into consideration. Combination of these groups with the Chelonia leads to some unknown forms whence also the Theromorpha have arisen. Palaeontology does not help us, all the leading, main groups of Chelonia having been in existence in the earlier Mesozoic ages, {313}and Palaeozoic Chelonia are still unknown. We can, however, to a certain extent, reconstruct an ideal primordial Chelonian by assigning to it all the ancestral characters actually observed in recent and fossil kinds, and by reducing to simpler conditions those features which we know to be more or less exaggerated specialisations. It is reasonable to assume that originally each metamere, except those of the anterior half of the neck and the posterior half of the tail, carried a transverse series of dermal plates, covered with horny shields, while the trunk, according to the greater bulk of the body, increased in size, converging towards the root of the neck and tail. By concentration, reduction of the number, and increase in the size of some of the remaining plates and shields, the skull assumed its characteristic box-like shape, the neck and tail becoming at the same time free. Chelonia are without doubt descendants of terrestrial, or at least semi-aquatic reptiles, and the marine paddled forms subsequently developed from terrestrial kinds.

CLASSIFICATION OF CHELONIA.–After many vicissitudes it was recognised that the Chelonia cannot naturally be divided according to the modification of their feet. The TRIONYCHOIDEA were clearly separated from the rest by Stannius in 1854. Cope, in 1870, was the first to emphasise the important character of the mode in which the neck is either bent sidewards (PLEURODIRA) or withdrawn in an S-shaped curve in a vertical plane (CRYPTODIRA); and he also separated _Sphargis_ as ATHECAE from all the other Chelonians, for which Dollo in 1886 proposed the term THECOPHORA. The division of the latter into recognisable families, based upon reliable, chiefly internal, skeletal, characters, has been effected by Boulenger;[127] and his classification has been adopted in the present volume, after intercalation of the more important fossil forms. The relationships between these various families may perhaps be indicated as follows:–

{ ATHECAE Sphargidae
{
{ { _Pleurodira_ { Pelomedusidae
{ { { Chelydidae–Carettochelydidae
{ {
CHELONIA { THECOPHORA { _Cryptodira_ { Chelydridae–Dermatemydidae–
{ { { Cinosternidae
{ { { Platysternidae
{ { { Testudinidae–Chelonidae
{ {
{ { _Trionychoidea_ Trionychidae

{314}The guiding taxonomic characters are fully mentioned at the head of the different families, and are mostly internal. The following "key," adapted from Boulenger, and based upon external characters, is preferable for practical purposes.

_For the position and names of the horny shields_ see Fig. 61 on p. 315.

Shell covered with horny shields.
Digits distinct, with 5 or 4 claws.
Pectoral shields separated from the marginals by inframarginals.
Tail long and crested. Plastron small and cruciform.
North America _Chelydridae_, p. 338.
Tail long, covered with rings of shields.
Plastron large. Indo-China _Platysternidae_, p. 345.
Tail short. North and { _Dermatemydidae_, p. 341.
Central America { _Cinosternidae_, p. 342.
Pectoral shields in contact with the marginals.
Plastral shields 11 or 12, without an intergular.
Neck retractile in an p. S-shaped vertical curve
_Testudinidae_, p. 345.
Plastral shields 13, an intergular being present.
Neck bending sideways under the shell
{ _Chelydidae_, p. 399.
{ _Pelomedusidae_, p. 390.
Limbs paddle-shaped, with one or two claws _Chelonidae_, p. 378.
Shell without horny shields, covered with soft, leathery skin.
Digits distinct, broadly webbed, but with only
three claws _Trionychoidea_, p. 404.
Limbs paddle-shaped.
Shell composed of regular series of
bony plates. Two claws _Carettochelydidae_, p. 404.
Shell composed of very many small plates arranged like
mosaic. No claws _Sphargidae_, p. 333.

The VERTEBRAE are, sometimes in the various regions of the same individual, amphi-, opistho- or pro-coelous, or even biconvex. Traces of the chorda remain longest in the middle of the centra. Intercentra occur regularly on the first two or three cervicals, and then again in the tail as paired or unpaired nodules, or as short chevrons. The latter occasionally fuse with the caudal end of their centra. Intercentral discs of fibrous cartilage occur regularly in the neck and tail. The ribs develop originally in the same transverse level with these discs, and frequently the anterior thoracic vertebrae retain this intercentral or intervertebral position throughout life. Farther back they often show a gradual change from the intercentral to a more central and ultimately {315}remarkable to a purely neural attachment. In all the Chelonia the ribs are devoid of the tubercular portion.

The cervical vertebrae have no ribs, except mere traces in the shape of small nodules. On the tail the ribs are often large, and, when fused with their neural supports, look like transverse processes; the whole arrangement exactly resembles that of Crocodilia. The first pair of thoracic ribs, those borne by the ninth vertebra, are peculiar. They arise from the anterior portion of the centrum, are much reduced, sometimes to mere threads of bone, and lean against the anterior rim of the second pair of ribs, in many cases without reaching the carapace. The next following ribs, those of the tenth to the sixteenth vertebra, are intimately involved in the formation of the first to seventh costal plates. The ribs of the two sacral vertebrae sometimes remain quite distinct throughout life, just touching the upper {316}ends of the iliac bones; but since these find a much more effective support in the shell, the distal ends of the sacral vertebrae fuse with the eighth, or so-called last, pair of costal plates.

The neural arch of the ninth vertebra rests upon its centrum; but the neural arches of the other trunk-vertebrae, although long, rest upon two centra; retaining, like the ribs, their original intercentral position; and in most cases the neuro-central sutures remain throughout life. The atlas and the last cervical vertebra deserve special attention. In many tortoises, _e.g._ _Trionyx_, _Clemmys_, _Testudo_, the three constituent parts of the atlas, namely, the neural arch, the centrum, and the intercentrum or first pair of united basiventralia, do not ankylose, but remain loosely connected; and the first centrum, instead of forming an odontoid process, remains movably attached to the second centrum, although it sometimes carries, and fuses with, the second intercentral piece. In other tortoises, _e.g._ _Platemys_ and _Chelys_, however, all the parts of the atlas co-ossify and form a complete, solid vertebra which articulates by a concavo-convex joint with the centrum of the second vertebra. The normal number of cervical vertebrae is eight in all Chelonians. The first spinal nerve issues between occiput and atlas, all the others behind the neural arches of their vertebrae. The last, or eighth cervical, owing to the retractility of the neck, forms elaborate joints; its centre fits with a knob into a cup of the ninth, and its post-zygapophyses form broad, curved articulating concave facets for the reception of the anterior zygapophyses of the fixed ninth vertebra. In the Trionychidae the zygapophyses are most elaborate, and they alone articulate with the ninth vertebra, while the centra do not join, but remain, or rather become, separated by partial resorption. In the Chelonidae, in conformity with the non-retractile and short neck, all the cervical joints are much reduced.

{317}

The SKULL (cf. Fig. 54, H, I, K, p. 280) agrees fundamentally with that of _Sphenodon_ and of the _Crocodilia_, but it is characterised by several special features. There are no ectopterygoids or ossa transversa; no lacrymal bones, no interparietal or pineal foramen; the vomer is unpaired and the nasal bones are mostly absent, unless they are fused with the prefrontals. The premaxillae are very small. The single vomer forms a septum between the choanae; and these are, except in _Sphargis_, ventrally roofed over by wings sent out by the palatines. The latter form a continuous bony roof to the mouth with the pterygoids, and these diverge posteriorly, being connected suturally with the quadrates, lateral and basi-occipital bones, and with the unpaired basi-sphenoid, which appears between the basi-occipital and the diverging pterygoids, but is in most cases to a great extent overlapped by the latter. The occipital condyle is distinctly triple; the basi-occipital sometimes helps to border the foramen magnum. The supra-occipital sends out a long vertical blade, directed backwards and generally projecting far over the neck, for the attachment of the powerful cranio-cervical muscles. The quadrate is very peculiar. Firmly attached, and hemmed in on nearly all sides by the neighbouring bones, it stands nearly vertically and forms a broad articulating surface for the mandible. Its posterior side shows either a transverse, horizontal groove, in which lies the columella auris, or the groove is transformed into a more or less closed canal. Moreover, the hinder lateral margin of the quadrate forms most of the tympanic frame; its margins being curved backwards, leaving in the Cryptodira, however, a {318}wide notch behind; in the Pleurodira this part of the quadrate is transformed into a trumpet, the wide rim of which, forming a complete ring, carries the tympanic membrane. The tympanic cavity thus formed often leads into a deep recess which extends beneath the squamosal towards the opisthotic and bears some resemblance to the intricate tympanic recesses which pervade that region of the Crocodilian skull.

Dorsally the quadrate is broadly overlaid by the squamosal, which frequently forms an arch with the parietal. Anteriorly the quadrate is connected through a variably sized quadrato-jugal with the jugal; and this, by joining the maxilla and postfrontal, helps normally to form the posterior rim of the orbit. All the bones which border the temporal fossa vary much in extent in the different groups of Chelonia. The extremes are represented by _Cistudo_ and _Geoemyda_, in which the bony infratemporal arch is absent, owing to the loss of the quadrato-jugal; and on the other hand by the Chelonidae and by _Sphargis_, in which the whole temporal region is covered over by an additional "false cranial" roof. This roof is produced chiefly by lateral wing-like expansions of the parietal and postfrontal bones, which meet the likewise much expanded jugal, quadrato-jugal, and squamosal bones. In the lower diagram of Fig. 63 (_Chelone mydas_) the squamosal has been removed, and the other bones have been reduced to their normal, or rather primitive condition, for comparison with the external view of the complete skull of the same animal. The lower diagram shows also the connexion of the pterygoid with a descending process of the parietal; this column, paired of course, usually contains a separate bone, the epipterygoid, the portion between _Ptg_ and _Par_.

The hyoidean apparatus is well developed, and sometimes assumes large dimensions, especially in _Chelys_. The two pairs of "horns" are the first and second branchial arches, whilst the hyoid arches are reduced to a pair of small, frequently only cartilaginous, nodules attached near the anterior corners of the basis linguae, which generally fuses with the os entoglossum in the tip of the tongue.

The PECTORAL ARCH consists of a pair of long coracoids sloping obliquely backwards, the distal cartilages of which scarcely touch each other in the middle line, and the scapulae. The upper end of the scapula frequently touches the inside of the {319}first costal plate, protected by a cartilaginous pad. Near the glenoid cavity arises a long process (PC in Fig. 65), placed transversely and approaching its fellow. The distal end is connected with that of the coracoid by a fibro-cartilaginous band. The homology of this scapular process is not quite clear. The band just mentioned favours the idea that the process represents the precoracoid, but its being an outgrowth from the scapula suggests that it is merely the much enlarged acromion. It certainly does not represent the clavicle, which forms part of the plastron: and this is not in contact with the shoulder-girdle at all.

The PELVIS is strong. Ilium, pubis, and ischium meet at the acetabulum. The dorsal end of the ilium is generally broadened, and is attached to one or both sacral vertebrae, but it is also in contact with the superimposed last costal plate. This additional connexion often becomes predominant and the sacral vertebrae are partly or completely relieved of the iliac support, fusing in this case more or less with the costal plates. The pubes have strong lateral processes, directed obliquely forwards and downwards. The pubes and the ischia, which latter are much smaller, form broad symphyses, and these are connected with each other by a longitudinal cartilaginous band (_Chelone, Trionyx_); or the connecting bridge is broad and quite ossified (_Testudo_), forming in the latter case two roundish obturator-foramina. Cartilage frequently remains at the anterior end of {320}the pubic symphysis, and a smaller, longer, and narrow piece of cartilage extends sometimes backwards from the ischiadic symphysis, as the so-called hypo-ischium. In the Pleurodira the ends of the ilia, and those of the lateral processes of the pubes, are much broadened and firmly ankylosed with the posterior costal plates and with the xiphiplastron respectively.

The LIMBS are typically pentadactyle and complete, and are most primitive in water-tortoises, e.g. _Chelydra_ and _Emys_, in which the carpus consists of the typical ten separate elements, including the pisiform. In _Testudo_ the centrale is fused with the intermedium, and the first three distal carpals are also fused together. In the marine turtles the limbs are transformed into paddles, but all the bones retain their independence; the pisiform {321}and the first metacarpal are enlarged and flattened, thereby giving additional width to the paddle. The tarsus remains less primitive; the centrale and the proximal elements have a tendency to fuse together, most completely in land-tortoises; the fifth distal carpal is enlarged, and stands out hook-like from the rest. The number of the phalanges of the fingers and toes varies slightly. It is noteworthy that none of the Chelonia possess more than three phalanges. The three middle fingers and toes have mostly three phalanges; the pollex and hallux have always two; the number of phalanges of the fifth finger varies from three to one, of the fifth toe from two to none. The greatest reduction occurs in _Testudo_ and its allied genera of typical land-tortoises, _Homopus_, _Pyxis_, and _Cinixys_, the formula for the fingers being 2, 2, 2, 2, 2 or 1, and 2, 2, 2, 2, 0 for the toes. In _Pelomedusa_ all the fingers possess two phalanges only, owing to fusion of the first and second phalanges with each other.

is indicated by dotted lines. _a_, Anal horny shield; _ab_, abdominal; _f_, femoral; _g_, gular; _h_, humeral; _ig_, intergular; _im_, infra-marginals; _p_, pectoral.]

The SHELL, which is the most characteristic feature of the Chelonia, consists of the dorsal "carapace" and the ventral "plastron." Each is composed of a considerable number of bony plates which arise as ossifications of nearly the whole thickness of the cutis, only a thin layer of subcutaneous connective tissue remaining soft and lining the inside of the shell. We restrict ourselves to a description of the shell of the Thecophora, leaving the discussion of the peculiar shell of _Sphargis_ to p. 336 f. Very young tortoises are still soft, and the plates which are beginning to ossify are not yet suturally united. The plastron (Figs. 66 and 67) consists of the paired epi-, hyo-, hypo-, and xiphi-plastral plates, and the unpaired endo-plastral plate.

{322}

The latter is homologous with the interclavicle, the epi-plastra are homologous with the clavicles of other Reptiles, while the other pieces are genetically derived from, and are further modifications of, the so-called abdominal ribs of the Crocodilia and Prosauria, These plastral plates are never in direct contact with the shoulder-girdle or with any other parts of the internal skeleton. In the young of all tortoises, and in the adult of the Chelonidae and Trionychidae, the several plastral plates enclose large, irregularly-shaped fontanelles. These are more or less filled up in the other groups; and in the Testudinidae especially the whole plastron forms one continuous mass. The navel is situated between the hyo- and hypo-plastrals. Both these pairs are broader than the others, and are connected with the carapace by {323}means of several marginals. The connecting region is called the bridge. In several tortoises, e.g. _Emys_, the connexion with the marginals is formed by ligaments only and remains movable. In others, transverse, more or less perfect hinges are formed across the plastron. A rather imperfect joint between the hypo- and xiphi-plastrals develops with age in _Testudo ibera_. In _Cistudo_ and _Cyclemys_ a very effective hinge lies below the hyo- and hypo-plastrals, just in front of the bridge; and the anterior and posterior lobes of the plastron can be closed against the inner rim of the box, fitting tightly in _Cistudo_. In _Pyxis_ the front lobe only is movable.

The carapace is composed of one median series, a right and left lateral series of costal plates, and a series of marginals which surround the whole. The median series consists of one large nuchal plate, normally eight neurals and one to three supracaudal plates. The characteristic feature of the neural plates is that they are firmly fused with the broadened neural spinous processes of the underlying vertebrae. The nuchal plate lies in front of the first thoracic or ninth vertebra; it overlies the last cervical vertebrae, with the eighth of which it is connected by ligament only; but the posterior corner of the plate often fuses with the spine of the ninth vertebra. In the Chelydridae, and still {324}more in the Trionychidae, the nuchal sends out a pair of long rib-like processes, which either extend to below some of the neighbouring marginals, or their ends overlap those of the ribs of the second thoracic vertebra (e.g. _Trionyx_), or, lastly, they are in turn overlapped by the first costal plates (e.g. _Cyclanorbis_). Such rib-like processes are also present, well developed in the young, shorter in the adult, in the Dermatemydidae and Cinosternidae. It is possible that the nuchal plate represents the fused neural of the eighth and the costal plates of the ninth vertebrae. An indication of the compound nature of the nuchal may be found in the fact that two nuchals have been described in _Chelydropsis carinata_, a Miocene relation of _Chelydra_. Somewhat similar modifications have taken place in the post-sacral region. The one to three supracaudal plates are, namely, neurals which have lost their connexion with, or perhaps have never been fused with, the spinous processes of the movable tail-vertebrae. The number of neural plates is mostly eight, but there are sometimes individually nine or ten, the gradual suppression taking place first in the sacral region. When such a plate is suppressed the neighbouring costal plates usually close up and meet in the median line. In _Cistudo_, for instance, there are only seven normal neurals, the eighth pair of costals meet, and the original eighth neural is transformed into a supracaudal. In _Cinosternum_ the sixth to eighth costals meet, separating the one supracaudal widely from the remaining five neurals. The meeting of the last pair of costals, with co-ordinate reduction of the neurals to seven, is almost universal in the Pleurodira; and this tendency is carried out to an extreme in the Brazilian _Platemys_ and in the Australian _Chelodina_ and its allies, in which all the costals meet in the middle line, and the neurals are completely suppressed. Every stage intermediate between complete neurals (_Sternothaerus_) and interrupted, vestigial, and vanished neurals, is still represented by some genus. This process takes place independently, both in America and in Australia, and is one of the most recently introduced modifications.

The costal plates arise, like the neurals, independently in the cutis, but they soon come into contact with the underlying cartilage of the ribs, which are long enough to reach the marginals. The ribs flatten, become surrounded by the growing membrane-bone of the plates, and the cartilage of the ribs, {325}instead of ossifying, undergoes a process of calcification. Ultimately this is more or less absorbed, its place is taken by the dermal bone, which forms so to speak a cast of the rib, preserving in many cases the shape of the vanished rib, only the capitular portions of which remain unaffected. The number of costal plates is very constant, namely eight on each side, but some fossils have nine or ten, and there are still individual variations in recent forms, indicative of that number. In a large _Chrysemys concinna_ I find the last pair of costals clearly composed of at least two pairs, and this same specimen has nine distinct neural plates.

The marginal plates are originally paired, almost always eleven pairs, very rarely ten or twelve; an unpaired posterior plate, the pygal, is always present, and is probably the result of fusion. In the Chelonidae large fenestrae remain between the costal and marginal plates, only covered by leathery unossified cutis, and of course by the horny shields. In the Indian fresh-water genus _Batagur_ similar windows are gradually filled up with age, and the horny shields become extremely thin and almost confluent. On the other hand, in _Testudo polyphemus_, the bony shell, always very thin, becomes still thinner with age and finally fenestrated by absorption.

Great reduction has taken place in the carapace of the Trionychidae. The American species of _Trionyx_ have only seven pairs of costal plates; in _Cyclanorbis_ the neurals are reduced to two. The whole dorsal shell is much smaller than the body, and marginal plates are absent or merely vestigial. It is doubtful if the ossifications in the posterior half of the marginal flap of some genera are homologous with true marginals.

Externally the whole shell is covered, except in the _Trionychidae_, in _Sphargis_ and _Carettochelys_ with horny, epidermal shields. These are phylogenetically older than the dermal plates, and they do not correspond with them either in numbers or in position, although there exists a general resemblance in their arrangement. On the plastron we distinguish an unpaired or paired gular, and a pair of gular, humeral, pectoral, abdominal, femoral, and anal shields (Fig. 66). Sometimes there are also intergulars, paired in _Macroclemmys_ and _Chelys_, unpaired in _Chelone_; in many of the Pleurodira an unpaired intergular lies behind the gulars.

{326}The carapace of most Chelonians is covered with five neural, four pairs of costal and twelve pairs of marginal _shields_, the last of which often forms an unpaired pygal. In front of the first neural lies the nuchal shield, very variable in size, often absent. The Chelydridae, Dermatemydidae, Platysternidae, and Cinosternidae possess moreover several inframarginals, intercalated on the bridge between the marginal and some of the plastral shields. In many of the other families these inframarginals are restricted to the anterior and posterior corners of the bridge, as the so-called axillaries and inguinals, mostly small and variable. Lastly, _Macroclemmys_ has several small supramarginals.

There are consequently eleven longitudinal rows of shields in all; by elimination of the supra- and infra-marginals they are reduced to seven rows. It is absolutely certain that the number of transverse rows also was originally much greater than it is now. The mode of reduction of the number of the neural and costal shields has been studied in _Thalassochelys caretta_ (cf. p. 388.) The accompanying illustration (Fig. 69) shows some of the main stages actually observed in the reduction of these shields. The chief point is that certain shields are squeezed out, or suppressed by their enlarging neighbours. The ultimate result is the formation of fewer, but larger shields.

Each shield grows individually as follows. Every year, or rather during every periodically recurring period of growth, the area of the Malpighian layer belonging to each shield increases peripherally in size, and at the same time produces a new layer of horn. The original little shield, with which the tortoise is born, remains for years, often throughout life, as the so-called "areola;" it increases in thickness owing to the new layer of horn added from below, and peripherally the increase in size is indicated by the overlapping concentric rings. Each ring represents a year's growth, at least in tortoises which live in temperate zones, where hibernation means a complete suspension of growth. It is not known if the same applies to tropical species, which grow either throughout the year, or which undergo one or more periods of rest. The areola does not remain central; the growth is uneven. With age the oldest layers of the areola are frequently rubbed off, and the areola then appears enlarged.

{327}), _e.g._ in several species of _Testudo_, or it is surrounded by the marginals (X_b_), _e.g._ in _Sternothaerus_. (From Willey's _Zool. Results_, 1899.)]

{328}For the first dozen years or so the annual rings can be easily followed, but when the creature approaches maturity each shield adds very little to its growth, and the rings become very fine, crowded and irregular. Only by careful counting and comparison of the rings on the costals, marginals, and plastrals, can a reliable average be arrived at. In some tortoises, e.g. _Chrysemys_, the whole outer layer of the shields peels off periodically; only a thin smooth layer like mica or tracing-paper remains, of course without any indication of rings. The pigment is formed in the Malpighian layer, but it frequently diffuses into the horny shields themselves, notably in _Chelone imbricata_, which yields the beautiful "tortoise-shell." The colour of the pigment is either black, yellow, or red, with resulting combinations. The green colour, often so beautiful in baby-specimens of _Chrysemys_, is optical, produced, according to Agassiz, by a network of black pigment, spread over a layer of yellow oil.

Horny scales, sometimes forming spines, and covering a nodule of dermal ossification, are also common on other parts of the skin, especially on the limbs of land-tortoises, and also on the tail of _Chelydra_. Sometimes the end of the tail is protected by a claw-like nail, for instance in _Pyxis_. In some of the gigantic land-tortoises, and in _Chelone mydas_, this nail assumes large dimensions, and several of the terminal caudal vertebrae are fused together into a regular urostyle. In some subfossil specimens of Mauritian tortoises, these ankylosed complexes are 12 cm. long and more than 5 cm. broad!

Before leaving the description of the shell, it is worth while to draw attention to the enormous correlative changes in other organs produced by this case. Nearly the whole organism has been altered. The hard, firm carapace has partly rendered the supporting functions of the vertebral column unnecessary or impossible. In many tortoises, especially in the large land-tortoises, the vertebrae and the capitular portions of the ribs are reduced to mere bony outlines; the reduction to thin paper-like bony lamellae proceeds with age. The iliac bones find a better support in the costal plates; the contact with the sacral ribs is given up, and these ribs fuse partly with the costal plates, or they are absorbed. The whole mass of muscles of the trunk is completely lost in the region of the shell, but traces of them exist in young specimens. Neck, limbs, and tail can in most cases be withdrawn and hidden in the shell. When this is not possible it is due to secondary changes. The neck is withdrawn either by being tucked away {329}sideways (Pleurodira[129]), or by being bent in an S-shaped curve in a vertical plane. In a left-sided profile-view of the animal, the head represents the tail of the S. The neck is withdrawn by long muscles, which are inserted into the ventral side of the middle of the neck, and extend in the shape of vertical ribbons far back into the shell, arising from the centra of some of the middle or even more posterior thoracic vertebrae.

Lastly, a few remarks on the PARTIAL REGENERATION, or the mending of injuries to the shell. If part of the horny covering is badly bruised, torn off, or rubbed through, or if part of the shell is crushed, the underlying portion of the bony plate becomes necrotic, and the horny covering also dies so far as its Malpighian layer is destroyed. Soon, however, the uninjured Malpighian cells, around the margin of the wound, multiply, grow into and beneath the injured portion of the bone, and form a new horny layer, casting off the necrotic portion. After several months the deficiency is patched up; new bone has grown in the deeper remaining strata of the cutis, and the outside is covered by a continuous horny layer, without, however, reproducing the original concentric moulding of the shields. In badly crushed shells sometimes almost one-third of the whole shell is thus cast off and mended within one or two years. The regeneration of the forcibly stripped-off shields of _Chelone imbricata_ is described on p. 386. Bitten-off tails and limbs, rather frequent occurrences in water-tortoises, are of course not reproduced, but the wounds are healed and covered again with scaly skin.

SENSE-ORGANS.–The EYE is by far the best developed sense-organ. It is comparatively small. The pupil is round. The iris is mostly dark in terrestrial forms, while in water-tortoises it is often brightly coloured, for instance pale yellow in _Chelodina_, greenish and mottled with black, pale grey, brown, etc., in various species of _Chrysemys_. _Cistudo_ presents a curious sexual dimorphism; the males have red, the females brown, eyes. The sclerotic wall contains a ring of numerous small ossified plates. There is no trace of a pecten. The eye is protected externally by the two lids and the nictitating membrane. In some water-tortoises, notably in _Chelodina_, the lower lid is transparent. Lacrymal and Harderian glands are present.

{330}The SENSE OF HEARING is apparently not very acute, although tortoises and turtles are frightened by noise, and can distinguish sounds; otherwise they would have no voice, which is very tiny and piping in most tortoises during the pairing season. In most water-tortoises the tympanic membrane is thin and quite exposed; in land-tortoises it is often thick and covered by the ordinary skin; lastly, in _Chelone_ the tympanic cavity is filled with a plug of the much-thickened skin, possibly in adaptation to the water-pressure when these creatures dive to considerable depths. The ossicular chain is mostly reduced to a long, bony, columellar rod.

The SENSE OF SMELL is well developed. All Chelonians carefully smell their food, in the air as well as under water. The individual predilection shown by many species for different kinds of animal and vegetable food,–since they are, for instance, able to distinguish between the various sorts of cabbage, cauliflower, sprouts, etc.,–proves that they possess a considerable amount of smell and taste.

Tortoises have a fine sense of touch; even the slightest tap on the shell is noticed, and the skin of the soft parts is extremely sensitive. Tickling of the sides of the tail, or of the hinder surface of a thigh, produces ridiculous scratching actions of the same or of the opposite foot.

The DIGESTIVE APPARATUS is simple. Only a few peculiarities need be mentioned. The tongue is mostly broad and soft; it cannot be protruded. The oesophagus of the Chelonidae is covered with many conical projections pointing towards the stomach. The latter is simple, except in _Sphargis_. The intestine is devoid of a caecum, but the difference between the small intestine and the rectum is very marked and often abrupt. The cloaca is very roomy. It contains the large copulatory organ, which is unpaired, grooved on its dorsal side, and is altogether constructed like that of the Crocodilia. The large bladder opens ventrally into the urodaeum, a recess of the cloaca; near its base open the urinary and genital ducts. Many water-tortoises possess also a pair of lateral thin-walled sacs, the so-called anal sacs, dorso-lateral diverticula of the walls of the urodaeum. These sacs, which have highly vascularised walls, are incessantly filled and emptied with water through the vent, and act as important respiratory organs. When such a water-tortoise, for instance an _Emys_ or a _Clemmys_, is suddenly taken out of the water, it squirts out a {331}stream of this water, which is not, as is generally supposed, the urine from the bladder.

The mode of RESPIRATION is interesting. The lungs are very complicated, highly-developed, spongy structures. They are attached by their whole dorsal surface to the inner lining of the shell. As they cannot expand through their own initiative, and since the shell has made costal and abdominal expansion impossible, the tortoise has to resort to other means of producing the necessary vacuum. This is done partly by the neck and the limbs, which act like pistons in being drawn in and out; partly by the greatly developed hyoidean apparatus, by which, when the neck is stretched out, the throat is alternately inflated and emptied, the air being swallowed, or pumped into the lungs. Additional respiration, besides that of the anal sacs mentioned above, is effected in various aquatic tortoises by slightly vascularised recesses of the pharyngeal region. Most Chelonians can exist for a very long time without breathing; sulky individuals remain for hours or days under water. _Cistudo_ can shut itself up for an equally long time. Nevertheless this and other land-tortoises easily get drowned.

All Chelonians lay white EGGS, round or oval, according to their kind, but the shape of the eggs of one set sometimes varies within the greatest limits. The shell varies from a parchment-like, flexible, scarcely calcareous cover to a hard, well-polished case. As a rule the eggs, imbedded in the ground, are hatched after a few months, but in some of the northern kinds, e.g. _Emys orbicularis_, the hatching is deferred until the next spring, the embryo's development being arrested during the winter. How such eggs, buried a few inches only below the surface, withstand the often very severe North German and Russian winter is a mystery. Whilst the plastron is generally flat, it is more or less concave in the males of many species, notably in _Testudo_, _Cistudo_, and _Emys_.

The general conclusions which can be drawn from the present GEOGRAPHICAL DISTRIBUTION of the Chelonia are as few and unsatisfactory as those applying to the Crocodilia, since all the main groups of Chelonians, and many more extinct families, occurred together in bygone ages in the same countries, for instance in Europe.

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The marine forms are naturally cosmopolitan, but the _Testudinidae_ are likewise cosmopolitan, except in the Australian region. The _Chelydridae_, now restricted to North and Central America, occurred formerly also in Europe. The _Pleurodira_, in Mesozoic times plentiful in Europe, India, and North America, are now restricted to South America, Australia, and Africa; the _Pelomedusidae_ to Africa, Madagascar, and South America; the _Chelydidae_ to South America and Australia. In the latter country all the Chelonians belong to the Chelydidae. The _Trionychoidea_, occurring since the Cretaceous epoch in North America, in Early and Mid-Tertiary times in Europe, are now restricted to North America, Asia, and Africa. The country richest in Chelonians is America; North and Central America together possessing representatives of all the families except the Pleurodira, and these we know to have died out there. The _Dermatemydidae_, {333}_Cinosternidae_, and Chelydridae are now restricted to the Nearctic sub-region (including Central America). Poorest in genera and species, all of them Chelydidae, is the Australian region, where no fossils of other families have yet been discovered. Europe, with its few Testudinidae, does not come into consideration; Asia has at least Testudinidae and Trionychidae, and in addition the solitary _Platysternum_ in Indo-China, representative of a family whose affinities with the Chelydridae again proclaim the validity of the Periarctic region.

ORDER I. ATHECAE.

_The vertebrae and ribs are not fused with, but are free from, the carapace, which consists of numerous small polygonal plates and is covered with leathery skin without any epidermal shields. The limbs are transformed into paddles. The neck is not retractile. Marine._

FAM. SPHARGIDAE.–_Sphargis_ s. _Dermatochelys coriacea_, the Leathery Turtle or Luth, is the only recent species and is the largest of all recent Chelonians. The biggest specimen in the national collection is about six feet and a half long, from the nose to the end of the shell, which latter is about four feet long; such a specimen may weigh half a ton. Agassiz, however, says that he has seen some "weighing over a ton." The general colour is dark brown, either uniform or with yellow spots. The Leathery Turtle has a wide distribution, ranging over all the intertropical seas, but it is rare everywhere; least so perhaps in the Western Atlantic from Florida to Brazil and in the Indian Ocean.

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According to Agassiz it breeds regularly every year in the spring on the Bahamas, on the Tortugas, and on the coast of Brazil, depositing its many eggs on the sandy shore like other turtles. Accidentally it visits the northern coast up to Long Island, and specimens, perhaps carried with the Gulf Stream, have been caught on the coasts of Europe, for instance off Dorsetshire. One was caught near Nantes in 1729, and is said to have made a terrible noise when being killed. This is perhaps the reason why Merrem in 1820 invented the generic name _Sphargis_, supposed to be derived from σφαραγέω (I make a noise). It has also been recorded from the Mediterranean. It seems to be entirely carnivorous, living upon Molluscs, Crustacea, and fish. The flesh is supposed to be unwholesome. It is a very curious fact that of this rare species only large specimens, besides a very few baby-turtles, are known or preserved in collections, while individuals of intermediate size, say from four inches to three feet in length, have never been recorded. If it were not for the fact that they are still known to breed, it would look as if the {335}species were dying out. Perhaps they are very shy, leading a pelagic life, diving at the least sign of danger, and coming near the land only for the sake of breeding.

The structure of _Sphargis_ is so peculiar in many respects that it deserves a somewhat full account. The neuro-central sutures persist on all the vertebrae. The eight cervicals are short. All the ten trunk-vertebrae carry ribs, and these, with the exception of the last, articulate between the centra and with the neural arches; the first and tenth ribs are short, the others are long and flattened, but not broad, with wide spaces between them. The tail is short, although it consists of about twenty vertebrae; these are devoid of chevrons.

The skull superficially resembles that of _Chelone_, chiefly owing to the completely roofed-in temporal region. The supraoccipital crest is rather short, covered completely by the parietals, the posterior margin of which is rounded off instead of forming, as in the Chelonidae, a long projecting triangular crest with the supra-occipital. The parietals are in broad contact with the postfrontals, posteriorly they are just reached by the squamosals. The quadrato-jugal is small, separated from the postfrontal by the meeting of the squamosal with the jugal. The quadrate is notched behind, and it separates the opisthotic from the squamosal. The basisphenoid is large and broad, extending far forwards so as to separate the pterygoids widely from each other except in their anterior portions, which, instead of sending a lateral arm to the jugal and maxillary, as in _Chelone_, are widely separated from these bones by the palatines. The choanae lie on either side of the anterior half of the vomer, and are not roofed over by ventral vomero-palatine wings.