496 ZOOLOGY. 



neither mammals nor snakes. The reptiles in cool climates 

 hibernate, while those of the tropics have a summer-sleep in 

 the dry season, becoming active when the rainy season begins. 



There are about three thousand species of living reptiles 

 known, of which three hundred and fifty-eight are North 

 American ; between three and four hundred fossil forms 

 have been described. The reptiles are divided into nine 

 orders, of which five are extinct. 



Order I. Opliidia. The snakes, of which there are over 

 one hundred and thirty species in America north of Mexico, 

 have a remarkably long cylindrical body, the tail very long 

 and slender ; they are footless, with no shoulder girdle, and 

 are covered with scales, which are all shed simultaneously. 

 These scales are epidermal growths, and while usually they 

 overlap, in a fewcases (Acroehordus, etc.) they are tubercu- 

 lar, and do not overlap. The eyes are not protected by true 

 lids, but the latter are thin, covering the eye permanently, 

 thus accounting for the fixed, stony stare of snakes. The 

 number of vertebrae (wliich are hollow in front and convex 

 behind), may in the boa amount to more than four hundred. 

 Each vertebra, except the first (the atlas) is provided with 

 ribs, and the processes with articular facets, which interlock- 

 ing give great strength and flexibility to the spinal column. 

 The hyoid bone is very slightly developed, though the 

 tongue is long, forked, can be rapidly darted out, and with- 

 drawn into a sheath ; the quadrate bones connecting the 

 lower jaw with the skull are movable. The bones of the 

 brain-case are firmly united together, while those of the jaws 

 and palate are more or less freely movable to allow the snake 

 (the boa especially) to distend its throat immensely and 

 swallow comparatively large animals, though ordinary snakes 

 will swallow large toads and frogs and other snakes but 

 slightly smaller than themselves. In order to retain the 

 prey and prevent its slipping out of the mouth, the recurved 

 short conical teeth are developed on the maxillary, palatine, 

 pterygoid, and mandibular bones, and occasionally on the 

 premaxillaries ; they are not set in sockets, and consequently 

 are not used to crush or tear food. 



The peculiar gliding motion of snakes is effected by the 



