Page:First course in biology (IA firstcourseinbio00bailrich).pdf/442

 (Geomys bursarius) × 1/4, a large, burrowing field rat, with cheek pouches for carrying grain.

—Hind foot a, fore foot b, tail c, of.

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 away by gnawing upon hard food, they become inconveniently long, and may prevent closing of the mouth and cause starvation. The hard enamel is all on the front surface, the dentine in the rear being softer; hence the incisors sharpen themselves by use to a chisel-like edge. 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