386 ZOOLOGY 



similar in shape but much larger. The turtles and snakes are 

 most widely different from the type and must be regarded as 

 much specialized, or even degenerate, forms. The turtles have 

 sought protection by means of a bony box, and are ill adapted 

 for motion either on land or water. Snakes, on the other hand, 

 elongated and devoid of appendages, are among the most rapid 

 and graceful of animals in their motions. The long tapering 

 body is a successful prehensile organ. Some of the lizards agree 

 with the snakes in lacking legs. 



416. Covering. The external covering in reptiles is in the 

 form of scales or plates formed by the epidermis, or the dermis, 

 or both. That deposited by the epidermis is horny and that 

 by the dermis, bony. In snakes and many lizards the scales 

 are epidermal and may be periodically shed and renewed. The 

 scales usually differ in shape and size in different parts of the 

 body. In turtles and their allies the horny constituent, which 

 is illustrated by the "tortoise shell" of commerce, is in the form 

 of plates and is reinforced by bony dermal plates beneath. 

 The latter do not, in the a'dult at least, correspond in number and 

 size with the former, but are closely associated with the bones 

 of the internal skeleton. In crocodiles the dermal scales cor- 

 respond in general with the epidermal. 



The members of the group are on the whole well protected 

 by these external growths. In many instances, as in certain of 

 the lizards, prominent projections are formed upon the body 

 covering, giving a striking appearance to the animals. 



417. Internal Skeleton. The vertebral column, except in 

 the snakes and snake-like lizards, shows the customary regions 

 (see 349). In the limbless forms only two regions are recog- 

 nized, the pre-caudal which bear the ribs, and the caudal or 

 tail vertebrae. The vertebrae are usually concave in front and 

 convex behind, thus making a kind of ball-and-socket joint. 

 In snakes the number of vertebrae is very large. No sternum 

 occurs in turtles and snakes. When present, as in lizards and 

 crocodiles, it is formed in connection with the ventral end of the 

 ribs. 



' The skull articulates with the first vertebra by one surface 



