442 



CHORD ATA. 



to the upper surface of the jaw (acrodont), or to the lateral 

 surface (pleurodont). The lizards are distmguished from 

 their nearest allies, the snakes, by the almost universal pres- 

 ence of four limbs, by the bones of the skull being immov- 

 able, and the mandibular rami being fused together. They 

 also have eyelids. Lizards are widely distributed, but found 

 in most profusion in equatorial regions. The common 

 slow-worm (with no limbs) and the sand lizard are British 

 examples. 



Fig. 312. — Lateral View of Skull of 

 Rattlesnake {Crotahis). 



Note the freely movable quadrate with pterygoid 

 continued into small palatine in front and joined to the 

 maxilla by a long transverse bone. Maxilla bears the fang. 



Order IIL — Ophidia, 



The snakes have an exoskeleton of epidermic scales. 

 They have no limbs, but progress by a movement of ventral 

 scales, to the inner surface of which the distal ends of the 

 numerous ribs are attached. Hence there is no sternum. 

 The vertebrae usually have extra articular facets (zygosphene 

 and zygantrium). The eyes have no eyelids. But in ad- 

 dition a unique method of locomotion, the snakes exhibit 

 a peculiar method of feeding. The quadrate is loosely 

 hinged on the skull, and the maxillae, palatines and pterygoids 

 are all freely movable. In addition, the mandibular rami 



