VERTEBRATA. 269 



most obvious features that distinguish Eep tiles from 

 Amphibia are the want of a larval stage and the pos- 

 session of one occipital condyle instead of two. 



Classification of Reptiles The Reptiles fall 



into three great groups, the Snakes, or Ophidia, cha- 

 racterized by the reduction of the limbs, and the 

 adaptation of the body to movement by crawling; 

 the Saurii, or Lizards, which usually have four limbs ; 

 and the Ghelonia, or Turtles, which have the body 

 invested by a continuous armour of bony plates. 



The Ophidia live on animal food of various kinds, 

 including insects, frogs, birds, and mice. They are 

 covered with an armour of scales, the colours of which, 

 dark brown and green for the most part, are such as 

 to render them inconspicuous, and thus help them to 

 steal unperceived on their prey. Their jaw-bones are 

 so constructed as to allow the mouth to open very wide, 

 and they can therefore swallow prey of comparatively 

 large size. The teeth of snakes are hooked and 

 conical. In addition to ordinary teeth, the poison 

 snakes have also poison teeth, which are grooved or 

 perforated by a canal, through which the poison from 

 the poison duct runs down when the snake bites. 

 These teeth are in the upper jaw. Snakes have no 

 trace whatever of the fore-limb or pectoral girdle, but 

 some of them have rudiments of the hind limb. They 

 have a great number of ribs, which play an important 

 part in helping them to crawl. Most snakes lay eggs, 

 but a few are viviparous. The Viper, Pelias berus, is 

 the only venomous snake found in England; it is 



