496 ZOOLOGY. 



neither mammals hor 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 Xorth 

 American; between three and four hundred fossil forms 

 have been described. The reptiles are divided into eleven 

 orders, of which sis are extinct. 



Order 1. Ophidia. The snakes, of which there are over 

 one hundred and thirty species in America north of Mexico, 

 have a remarkably long cyUndrical body, the tail very long 

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

 are covered with scales, which are all shed simultaneously. 

 These scales are epidermal growths, and whUe usually they" 

 overlap, in a few cases {Acrochordus, 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 accountiug for the fixed, stony stare of snakes. The 

 number of vertebrse (which are hoUow 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 oat, 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 firmlyunited 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, aud occasionally on the 

 premaxillaries ; they are not set in sockets, and consequentlv 

 are not used to crush or tear food. 



The peculiar gliding motion of snakes is effected by the 



