CLASS MAMMALIA 



659 



in colonies of from forty to one thousand. They feed upon grass 

 and other vegetation. The woodchucks, or ground-" hogs " 

 (genus Marmota), also live in burrows; but are usually not 

 colonial, and prefer hillsides or pasture land for their homes. 

 They feed on clover and other grass. The flying squirrels 

 (genus Sciuropterus) are delicate nocturnal rodents that spend 

 the day asleep in a nest, usually in a cavity in a tree. They 

 possess a thin fold of skin between the fore and hind limbs on 



Fig. 522. — The chipmunk, Tamias strialus. (From Ingersoll.) 



either side, which, when spread out, acts like a parachute to 

 sustain the animal in the air. 



The beavers (Castorid^e) are the largest gnawing animals in 

 North America. They are adapted for life in the water, pos- 

 sessing webbed hind feet and a broad flat tail. The dams of 

 wood, grass, and mud made by beavers are constructed for the 

 purpose of forming ponds in which houses are built with under- 

 water entrances. 



The pocket-gophers (Geomyid^e) possess large cheek-pouches, 

 which open outside of the mouth, and strong fore feet provided 



