MAMMALIA 



127 



numbers of which inhabit the plains on the east of the Rocky- 

 Mountains (fig. 87). 



Beavers are comparatively large Rodents which spend a large 

 part of their time in the water and are modified accordingly, the 

 hind-feet being webbed, and the rudder-like tail flattened and 

 scaly. There are only two living species, one in Eurasia and 

 the other in North America. The former, or European Beaver 



Fig. 87. — Prairie-Dogs {Cynomys Ludomcianus) 



{Castor fiber), at present ranges from France to Siberia, but was 

 once distributed much more widely, and in remoter times was a 

 member of the British fauna, as attested by numerous remains. 

 The American Beaver (Castor Canadensis) is now practically 

 limited to Canada, though it once extended over the greater 

 part of North America. 



3. Mice include a very large number of species, and are 

 represented in all parts of the world. A characteristic anatomical 

 feature is found in the union of the two bones of the lower leg 

 (tibia and fibula). The group includes the following families: — 

 (i) Mice and Rats; (2) Mole-Rats; (3) Pouched Rats; (4) Jump- 

 ing-Mice; and (5) Dormice. 



(i) The Mouse and Rat Family includes more species than 

 the others, and unlike them is cosmopolitan, being represented 



