332 



SAUROPSIDA. 



instance, the Common English Adder, which normally retains 

 the ova in the uterus until they are incubated. 



Many Eeptilia are provided with numerous teeth, some being 



connected with poisonous glands 

 at their base (fig. 171, d). The 

 poison-fangs have a tube run- 

 ning up them, through which 

 the poison is ejected when the 

 snake bites. Teeth of reptiles, 

 except in the Crocodiles (fig. 

 170, i and ii), are not lodged 

 in sockets, as we see is the case 

 in the jNIammalia, but are im- 

 planted in one long groove near 

 the edge of the jaw. 



There are four existing 



the Chelonia, or Tortoises and 



the Ophidia, or Snakes; 



Fig. 170. — Pleurojjont and Acrodoxt 

 Dentition. 



i, Pleurodont dentition (in crocodiles, 

 &c.); ii, acrodont dentition (note teeth 

 ancliylosed to the bone). (Brit. JIus. 

 Guide.) 



groups of Eeptiles — namel}'-. 

 Turtles ; the Lacertilia, or Lizards 



Fig. 171. — Head of Reptiles. 



A, He.id of adder ; B. of grass-snake ; C, side view of A ; P, jioison fangs ; T, tongue. 

 Dj poison-bag atid fang. 



and the Crocodilia, or Crocodiles and Alligators. Numerous 

 extinct forms are found in the Mesozoic or Secondary rocks, 

 a period of the earth which was characterised by the abundant 



