124 



VERTEBRATA : MAMMALIA. 



FIG. 138. 



The Rodents are readily distinguished by their teeth. 

 In each jaw they have two chisel-shaped 

 incisors, between which and the molars 

 there is a wide space without teeth, ca- 

 nines being wanting. The incisors are 

 covered with enamel only in front, so 

 that their posterior edges wear away 

 ami teeth oi a faster than the anterior edges, thus al- 

 ways keeping these teeth sharp, however 

 much they are used ; and they grow at the base as fast as 

 they wear away at the summit. The lower jaw is articulated 

 with the skull in such a manner that the jaws have no hori- 

 zontal motion, except backwards and forwards as is requi- 

 site in the act of gnawing. The enameled ridges of the 

 molars are transverse, thus in opposition to the horizontal 

 forward and backward motion of the jaw, and exactly 

 adapted to the process of trituration. The form of the 

 Uodentia is generally such that the hind parts consider- 

 ably exceed the forward parts ; most kinds are thus adapt- 

 ed to leaping instead of walking. The brain of the 

 Rodents is smooth (Fig. 66), that is, without convolutions. 

 Some kinds of the Rodentia, as the Jerboas or Dipodidre 



FIG. 130. 



Fro. 140. 



Jumping Mouse. Jaculus hudsonius, 

 Baird. 



White-footed Mouse, Hesperomys leucupua, 

 Wagner. 



of Asia and Africa, and the Jumping Mice or Jaculidse 

 (Fig. 139), of this country, have the tail and hind legs 



