REPTILIA 387 



and alligators are similar in shape but much larger. The tur- 

 tles and snakes are most widely different from the type and 

 must be regarded as much specialized forms. The turtles have 

 a characteristic protective bony box, and are ill adapted for 

 rapid motion either on land or water. Snakes, on the other 

 hand, elongated and devoid of appendages, are graceful in their 

 motions and some species exhibit rapid locomotion. 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 homy 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 homy 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 adult 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 iwo regions arerecog'-' 

 nized, — the pre-caudal vertebrse which bear the ribs, and the 

 caudal or tail vertebrse. The vertebras 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 iii 

 lizards and crocodiles, it is formed in connection with the ven- 

 tral end of the ribs. 



The skull articulates with the first vertebra by one surface 



