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 eleven 
orders, of which six 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 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 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 accounting for the fixed, stony stare of snakes. The 
number of vertebrae (which are hollow in front and convex 
behind), may in the boa amount to more than four hundred. 
Hach 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 
* See Garman’s Reptiles and Batrachians of N. Am., 1888; also 
Baird, Cope, etc. 
