390 ELEMENTARY ZOOLOGY 



The special arrangement and character of the teeth are 

 characteristic of this order. There are no canines, a 

 toothless space being left between the incisors and molars 

 on each side. There are only two incisor teeth in each 

 jaw (rarely four in the upper jaw), and these teeth grow 

 continuously and are kept sharp and of uniform length by 

 the gnawing on hard substances and the constant rubbing 

 on each other. The food of rodents is chiefly vegetable. 



Of the hares and rabbits the cottontail (Lepus mittalii) 

 and the common jack-rabbit (L. campestris) are the best 

 known. The cottontail is found all over the United 

 States, but shows some variation in the different regions. 

 There are several species of jack-rabbits, all limited to the 

 plains and mountain regions west of the Mississippi River. 

 The food of rabbits is strictly vegetable, consisting of suc- 

 culent roots, branches, or leaves. Rabbits are very 

 prolific and yearly rear from three to six broods of from 

 three to six young each. There are two North American 

 species of porcupines, an Eastern one, Erethison dorsatus, 

 and a Western one, E. epixanthus. The quills in both 

 these species are short, being only an inch or two in 

 length, and are barbed. In some foreign porcupines they 

 are a foot long. They are loosely attached in the skin 

 and may be readily pulled out, but they cannot be shot 

 out by the porcupine, as is popularly told. The little 

 guinea-pigs (Cavia), kept as pets, are South American 

 animals related to the porcupines. 



The pocket gophers, of which there are several species 

 mostly inhabiting the central plains, are rodents found 

 only in North America. They all live underground, 

 making extensive galleries and feeding chiefly on bulbous 

 roots. The mice and rats constitute a large family of 

 which the house-mice and rats, the various field-mice, the 

 wood-rat (Ncotoma pennsylvanicd) and the muskrat {Fiber 

 zibet kicus) are familiar representatives. The common 



