MAMMALS 327 



in length. These animals are active in the water, but 

 on land they are well-nigh helpless and can be easily 

 approached and killed. They inhabit the northern At- 

 lantic and Pacific oceans and are of great use to the natives 

 around Behring Strait. The flesh is used for food, the skins 

 for roofs of houses, for dog harnesses, and for fishing lines. 

 The tusks are used for making implements and are sold in 

 trade. 



The Gnawing Mammals (Rodentia) 



The members of this order of mammals are usually small in 

 size and possess a furry {sometimes spiny) covering. The 

 canine teeth are absent, but the incisors are long, v/iih chisel- 

 like edges and grow from the roots as fast as they are worn 

 down by gnawing. 



Technically, these animals are known as the Rodentia, or 

 rodents, — both words being derived from the verb rodo, 

 gnaw. In this group are included the rat, mouse, rabbit, 

 woodchuck, squirrels, etc. 



The teeth of the rabbit have already been described and 

 their fitness for gnawing has been explained. The rat, 

 which, perhaps, is a more typical gnawer, has only two in- 

 cisors on the upper jaw, but they meet the incisors on the 

 lower jaw in the same manner as do those of the rabbit and 

 are kept sharp in the same way. Keeping, then, the general 

 form and peculiarities of a rodent's teeth in mind, as exem- 

 plified by those of a rat and rabbit, we shall pass to others 

 of the group. 



Porcupines. — These animals are also gnawers, and the 

 species found in the United States are more or less aboreal 

 (Fig. 215). The Old World porcupines are strictly ter- 

 restrial. Porcupines are covered with coarse hairs among 



