704 MAMMALIA [CH. 



recent years however this latter species has increased its numbers 

 and it is now holding its own. The Common British Mouse is 

 M. musculus : the Wood-mouse, M. sylvaticus, and the Field-mouse, 

 M. minutus, also occurs in Britain. The Water-rat or Vole, Arvicola, 

 is distinguished from the true Rat by the fact that the cusps on its 

 back teeth, instead of being rounded as in the true Rat, are angular. 

 A. ampkibius, the Water-vole, A. agrestis, the Field- vole, which 

 often does much damage to crops, and A. glareolus, the Bank- vole, 

 represent the genus in Britain. The Dormouse, Muscardinus avella- 

 narius, which like the Squirrel passes the winter in a hole in a tree, 

 has a long bushy tail, and, in outward appearance at any rate more 



FIG. 355. The Musquash, Fiber zibethicus. 



resembles a tiny Squirrel than a Rat. In its skull it resembles the 

 MURIDAE, but it differs from both Squirrels and Rats in not 

 possessing a caecum on the intestine. On this account it, along with 

 five or six allied species from Europe and Africa, has been separated 

 as a distinct family, the MYOXIDAE. 



Amongst the most interesting American Rodents are the Beaver, 

 the Porcupine, the Ground-squirrel, the Marmots and the Musquash. 

 The Beaver, Castor canadensis, sole representative of the family 

 CASTORIDAE, has a broad flat tail, suited for swimming, which is 

 covered with horny scales. The Beaver, by means of its sharp 

 incisors, cuts down trees growing on the banks of streams, so that 

 they fall across streams thus damming them up and raising the 



