332 



SAUEOPSIDA. 



instance, the Common English Adder, which normally retains 

 the ova in the uterus until they are incubated. 



Many Eeptijia 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 Mammalia, but are im- 

 planted in one long groove near 

 the edge of the jaw. 



There are four existing 

 groups of Reptiles — namely, the Qhelonia, or Tortoises and 

 Turtles ; the Lacertilia, or Lizards ; the Ophidia, or Snakes ; 



Fig. 170. — Pleurodont and Acrodokt 

 Dentition. 



i, Plevirodont dentition (in crocodiles, 

 &c.); ii, acrodont dentition Qiote teeth 

 anchylosed to the hone). (Brit. Mus, 

 Guide.) 



A 



FiQ. 171. — Head of Reptiles. 



A, Head of adder ; B, of grass-snake ; C, side view of A ; P, poison fangs ; T, tongne. 

 Df poison-bag and fang. 



and the OrocodiKa, 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 



