FIELD BOOK OF MAMMALS 



The Norway Rat is aggressive and so easily adapts itself to 

 varying conditions that today it has a cosmopolitan distri- 

 bution, and drives out our native Rats wherever it meets 

 them. Living in great numbers in all our large cities and 

 water-fronts, it is also found widely spread over less inhabited 

 districts, frequenting fields and bushy areas where it finds 

 favorable conditions. The Norway Rat, with its coarsely 

 annulated tail, harsh pelage, and ashy underparts, only 

 superficially resembles any of our native species and should 

 be easily distinguished from the Wood Rats (which it most 

 resembles) upon a close examination. 



The Black Rat and the Roof Rat are so rare in most places 

 that they will not lead to confusion with native Rats. The 

 color and texture of pelage (coarser than in the Wood Rats), 

 and long, nearly naked tail, are the best field characters. In 

 tropical America these two introduced Rats are quite common, 

 but throughout most of the United States they have been 

 driven out by the Norway Rat. These two Rats, especially 

 the Roof Rat, are more attractive in appearance than the 

 Norway Rat. 



All three of the introduced Rats display in the three rows 

 of tubercles on the molar crowns an unmistakable character 

 of separation from all New World Rats. 



Family Aplodontiidae. Mountain Beavers 



Burrowing rodents of medium to large size; form robust; 

 legs rather short; tail vestigial; feet with five toes; tibia and 

 fibula distinct; skull massive, widened posteriorly, constricted 

 interorbitally ; molar teeth simple. 



Genus Aplodontia^ 



Dentition: Incisors, f ; Canines, §; Premolars, f ; Molars, f =22. 



Mountain Beaver. — Aplodontia rufa 



and related forms 



Names. — Mountain Beaver; Showtl; Sewellel; less fre- 

 quently Mountain Boomer, Ground-hog, Woodchuck. Plate 

 XX. 



^ For a full revison of this genus see Walter P. Taylor, University of 

 California Publications, Zool., Vol. 17, pp. 435-504, 1918. 



