Characteristic features of these inhabitants of the desert are the following:–
1. Velocity. The Lizards are slender. The Sand-snake, Tephrometopon, is whip-like; even the Cobra has a relatively narrower and longer tail than the Indian specimens, although the number of the vertebrae and of the scales is the same. All the desert-snakes are remarkable for the great number of their ventral shields, two hundred and more.
2. Hard, scaly covering, for instance in Agama, Echis, Gymnodactylus, Teratoscincus; the latter with its fish-like scales is exceptional among Geckos, resembling the likewise deserticolous Geckolepis and Homopholis of Africa.
3. Capacity for digging in the sand in order to escape great cold, or burning heat. All the Lizards and the Tortoise, Testudo horsfieldi, have strong claws. The snakes Typhlops and Eryx dig with their specially modified snouts, and their tails are very short and blunt. The Sand-viper, Echis, has the scales of the back arranged in very oblique rows, so that it can heap sand upon its body by wriggling, shaking, and up-and-down motions of the body. The Agamoid Phrynocephalus does this by means of lateral folds of the skin.
4. Arrangements for running on sand. The lizard Eremias has very large crural shields; Scapteira has the digits broadened out into shovels; others, e.g. Phrynocephalus and Teratoscincus, have long lateral fringes on the digits, a very rare arrangement among Geckos, occurring elsewhere among them only in Ptenopus and Stenodactylus, which are likewise inhabitants of the desert.
5. Protection against the everlasting, ubiquitous sand. In the digging species the nostrils are directed upwards instead of forwards; in most of the snakes they are protected by complicated valves, or they are reduced to small pin-holes. The eyes of Typhlops are overhung by the head-shields. In Agama and Phrynocephalus the margins of the lids are broadened into plates and are furnished with peculiar scales. In Teratoscincus the upper lid is enlarged. The lizard Mabuia has the lower lid much enlarged, with a transparent window in it, so that the eye can be closed without impeding sight, an arrangement carried to the extreme in Ablepharus, cf. p. [560]. The ear-opening is either small, or protected by fringes of scales, or it is abolished, e.g. in Phrynocephalus.
6. Coloration. Pure green is quite absent, even in Bufo viridis and in Rana esculenta, since there is no green in that country, at least not of long duration. White, with grey and black spots, occurs only in the nocturnal Geckos. Yellow, brownish, reddish colours are common, in adaptation to the sand. The advantages of the carmine-red, and of the blue spots of Phrynocephalus, and the yellow or bright red under surface of its tail, are unknown. Striation is of frequent occurrence among the lizards and snakes, probably in adaptation to the dry grass heaped up around the scattered shrubs.
Concerning the various organic systems of the Autosauri only some of the more important features may here be mentioned.
Skeleton.–The vertebrae are procoelous, with the exception of most of the Geckones, in which they are amphicoelous. So-called intercentra, in the shape of unpaired nodules or wedges, persist between most of the cervical vertebrae. In the tail these wedges, the remnants of the basiventralia, are generally present, frequently in the shape of chevron-bones. Sometimes they fuse with the centra of the vertebrae; occasionally the axial or central portion of these basiventrals persists as a sort of fibrous disc, which may calcify separately, and is interposed between the caudal end of the centrum and the articulating knob. The caudal vertebrae of the Geckones and of most Lacertae are liable to break across, like those of Sphenodon. They are enabled to do this owing to a transverse split, which makes its appearance with the ossification of the vertebral bodies and extends later into and across the neural arch and the various lateral processes. The split is ultimately referable to a transverse septum of cartilage, wrongly called chordal cartilage, which develops in the shell of the body of the vertebra, destroys the chorda, and extends peripherally. The cells of this septum retain throughout life their juvenile quasi-embryonic character. When the tail is broken off–and this always happens at such a septum–the cells of the remaining half reproduce a new tail. The latter is, however, in reality a sham tail, since neither new centra nor arches, but only a non-segmented rod or tube of fibro-cartilage is produced by this process of regeneration. Reproduction of centra is precluded by the previous normal reduction of the chorda, around which alone proper bony centra could be formed. The regenerated tail is, however, invested with new muscles, and with skin, but the scales often differ considerably from those of the normal organ. Boulenger[[148]] has found that the new or aberrant scaling is in some cases a reversion to an ancestral form. This is, for instance, the case in Pseudopus, and in the Tejoid genus Gymnophthalmus; to a certain extent also in Geckos and Skinks. On the other hand, Lacertidae, Gerrhosauridae, and also Anguidae reproduce a caudal scaling true to their type. Injured or broken-off tails are often reproduced double, or even trifid; sometimes an additional little tail grows out from an injured spot, anywhere on the side of the old remaining but mended tail.
The ribs of the trunk articulate by their capitula only, while the reduced tubercula are attached to their vertebrae by ligaments. In the tail the capitular portion is much reduced, while the tuberculum is much stronger and lies behind, no longer above, the capitulum, fusing sometimes directly with the centrum. The ribs of the poststernal region of Geckos and Chameleons are very long, and meet each other in the middle line, forming thin cartilaginous hoops.