352 BRANCH CHORDATA 



fibula are distinct. They feed upon nuts and grain, as well as eggs and 

 young birds. They are chiefly arboreal, building their nests in tree-tops. 

 They lay up a store of food in hollow trees, where they pass the severe 

 winter weather. Four species of this genus (Sciurus) are found in the 

 United States and Canada. S. vulgaris, the common squirrel of Great 

 Britain, is found from Ireland to Japan. 



Flying squirrels (genus Sciurop'terus) (Fig. 284) of the palsearctic region 

 (which includes Europe, northern Asia and Japan, North America, and 

 India) have a furry membrane connecting the anterior and posterior limbs. 

 This, together with the broad tail, acts as a sort of parachute, enabling these 

 squirrels to take enormous downward leaps from limb to limb or tree to 

 tree. They cannot "fly" upward, but ascend the tree by climbing. The 

 Asiatic flying squirrel is 16 to 18 inches long without the tail, and, it is said, 

 80 yards have been covered in their longest leaps. 



The little striped ground squirrel {Tamias striatus) burrows and carries 

 its food in its check i)oiiclies to its nest in the ground. 



The prairie-dog (genus Cy'nomys), of the great western plains, is also sub- 

 terranean, digging a burrow and throwing up a mound at its entrance. 

 There are whole villages of these mounds, sometimes covering acres. The 

 prairie-dog hibernates in winter, at least comes out only occasionally. One 

 kept in a cage by the author comes out of his "straw burrow" at night or 

 just before dawn, only at long intervals, for food. They sit up on their 

 hind feet and look all around like sentinels, but dart back into their burrows 

 again at the least approach of danger, uttering a shrill cry as a warning to 

 the rest of the community. They are so quick of movement that it is diffi- 

 cult to shoot or trap them. Their ears are small. Their legs are so short - 

 that in running they "hug the ground," of which, in sandy regions, they are 

 very nearly the color. They grasp their food with their paws like true 

 squirrels. 



The marmots of the arctic regions are closely allied to these. The Alpine 

 marmot {Ardo'mys marmotta) lives far up in the Alps. Its danger signal is 

 a shrill whistle. Th(>8e inannots hibernate, ten to fifteen being packed 

 together in a well-lined burrow. 



The North American beaver (family Castor'idce) is an aquatic rodent with 

 a stout body, flat, scaly tail, and webbed hind feet. It fells trees by means 

 of its strong incisors, damming the stream so as to raise the level of water 

 above the entrance to its burrow. The beaver (Castor canadensis) is fast 

 becoming exterminated on account of the demand for its fur. 



Rats and mice {Muridce) have naked tails which are scaly underneath. 

 The soles of the feet are naked and the tibia and fibula are united below. 

 Some of the numerous genera are found in all parts of the world. The 

 muskrat is the largest member of this family. Some of the species are 

 among the smallest quadrupeds known . 



The North American muskrat (genus Fiber) is a genus of two species of 

 dark brown aquatic animals. They dig burrows in the banks of streams, the 

 entrances to which are beneath tiie surface of the water. The hind feet 

 are slightly webbed, the tail flalieiieil and scaly, with scattered hairs. 

 The shortened thumb has a fully (le\ eloped claw, and they grasp theii food, 

 which consists of roots and water plants, with their paws, like the squirrels. 

 A Rocky Mountain species [Filter osoyoosensis) is said to construct a dome- 

 shaped house of "buhnishes" in the water.' It feeds largely upon water- 

 fowls and fish. 



The common mouse (Miis mus'culus) and rat {M. decumanus) have 

 been introduced into America from the Old World. M. minutiis is said to 



