3 2 4 THE ANIMALS OF ARCTIC AMERICA AND CANADA 



high enough to leave room for an air-chamber, usually connected with one or more 

 of the outlets which serve as exits for the owners when in search of food. To a 

 great extent these structures are used more as store-rooms than as dwelling-places, 

 but at times they contain the nest, though, as a rule, this is placed in a burrow. 

 Generally this burrow includes a single chamber, reached by a passage of a few 

 yards in length, which opens under water. 



A distinctive group of North American rodents is formed 



Pocket-Gophers. ^ ^ so-called pocket-gophers, whose range southwards does not 



extend farther than Central America. They take their name from the presence of 



a pair of cheek-pouches, which open outside the mouth on the lower edges of the 



cheeks. Two well-known northern representatives of the group are the Hudson 



v 





CANADIAN PORCUPINE. 



Bay pocket-gopher (Thomomys talpoides), ranging through Canada to the Missouri 

 district, and the common pocket -gopher {Geomys bursarius), whose habitat 

 extends from the Canadian border southwards to Kansas, Missouri, and Illinois. 

 In general form these gophers resemble ordinary mice and rats. There are, 

 however, other allied American rodents commonly known as kangaroo-rats, which 

 also have pouches, but hop on their hind-legs after the manner of the rat- 

 kangaroos and jerboas. Scientifically these are known as Dipodomys, Perodipus, 

 Heteromys, etc. 



Canadian According to modern ideas of classification, another exclusively 



Porcupine. American family of rodents is typified by the Canadian porcupine 



(Erethizon dorsatus), which is widely distributed in North America, ranging as far 



northwards as the limit of trees. On the eastern side of the continent it reaches as 



