204 



ANIMAL BIOLOGY 



except the rabbits, which have six (see Fig. 345). They 

 are readily recognized by their large incisors. These teeth 

 grow throughout life, and if they are not constantly worn 



Fig. 371. — Pouched Gopher {Geomys bursarius) xj,a large, burrowing 

 field rat, with cheek pouches for carrying grain. 



away by gnawing upon hard food, they become incon- 

 veniently long, and may prevent closing of the mouth and 

 cause starvation. The hard enamel is all on the front sur- 

 face, the dentine in the rear being softer ; hence the in- 

 cisors sharpen themselves by use to a chisel-like edge. 



Fig. 372. — Hind foot a, fore foot b, 

 tail c, of Beaver. 



Fig. 373. — Beaver. 



The molars are set close together and have their upper 

 surfaces level with each other. The ridges on them run 

 crosswise so as to form a continuous filelike surface for 



