494 LACERTILIA 



has very large crural sliields ; Scapteint has the digits broadened 

 out into sliovels ; 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 Ptenopiis 

 and Stenodactyhcs, 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 compli- 

 cated valves, or they are reduced to small pin-holes. The eyes 

 of Typhlo})S are overhung by the head-shields. In Agama and 

 Phrynocephalas the margins of tlie lids are broadened into plates 

 and are furnished with peculiar scales. In Teratoscincus the 

 upper lid is enlarged. The lizard Mabida 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 Ahlepharus, cf. p. 560. The ear-opening is either 

 small, or protected by fringes of scales, or it is abolished, e.g. 

 in Phrynoceplialus. 



6. Coloration. Pure green is quite absent, even in Bufo 

 viridis and in Rana escidenta, 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 connnon, in adaptation to the sand. 

 The advantages of the carmine -red, and of the blue spots of 

 Phrynoceplialus, 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 



