2O6 



BACKBONED ANIMALS. 



of the large ventral or lower scales. The skin is moulted 

 once a year, the process being assisted by the growth of 

 casting-hairs (Fig. 247) beneath the skin, that push it 



FIG. 246. Skeleton of a snake, sp, spinous processes of the joints ; r, ribs ; 

 <?, quadrate bones, joining upper and lower jaws ; e, front of the lower 

 jaw, where there is an elastic band in the place of bone ; , ball end of 

 joint, facing the tail ; c, cup end of joint, facing the head. 



upward. In the North 

 they hibernate in win- 

 ter, and in the South 

 pass through a summer 

 sleep. Over one thou- 

 sand species are known, 

 one hundred and thirty 

 species being found in 

 North America. 



The Vipers ( Vi- 

 psrida). About twenty- 



FiG. 2 47 .-Casting process in the skin two S p ec ies of these 



of the adder. // //, the casting hairs ; 

 the portion of skin above them is 



snakes are known. In 



pushed away, and the hairs themselves the puff-adder, of the 

 form the ridges on the new skin. Cape of Good Hope, the 



