432 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



size, and the general structure of the skull indicates great incisive power, the 

 Beavers being "gnawers" par excellence. 



The living representatives of the family Castor idee belong to the single 

 genus Castor. Beaver-like animals, belonging to five or six other genera, 

 have, however, been referred to the same group. Some of these (Steneojiber, 

 including Palctocastor Leidy and Chalicomys) differ widely from Castor, while 

 one (Cast oroides) is more nearly related to the Chinchillas (Lagostomus) than 

 to any other family of Rodents. Others (Trogontherium and Eucastor) evi- 

 dently are closely related to Castor. The extinct genera, above named, are 

 as yet known from too scanty materials to render certain their true affinities. 

 Trogontherium, whose remains occur sparingly in the Tertiary deposits of 

 Europe, evidently greatly resembled Castor ; it was, however, fully one-fifth 

 larger, and in its dentition differs generically from Castor. The genus Eucas- 

 tor, known as yet from very imperfect cranial remains found in the Mauvaises 

 Terres of Dakota, of the size of a Marmot (Arctomys), seems to have been 

 a true Beaver, apparently as closely related to Trogontherium as to Castor. 

 Chalicomys and Steneojiber differ considerably both in dentition, and in the 

 general form of the skull, from the true Beavers, and may prove, when better 

 known, to pertain to an entirely different group. Chalicomys has thus far been 

 found only in the Tertiary formations of Europe, while Steneojiber had rep- 

 resentatives not only in Europe, but in the Tertiary formations of New 

 Mexico and Dakota. 



The genus Castoroides, heretofore always referred to the Castor idee, has, 

 as already shown,* only a superficial resemblance to the true Beavers. 



Genus CASTOR Linn. 



Castor Linn., Syst, Nat., i, 1766, 178. 



Chaks. — Feet four-toed ; hind feet palmate, with the second toe double- 

 clawed. Tail broad, flat, and scaly. Body stout and heavy, most strongly 

 developed posteriorly. Grinding-teeth %=%, single-rooted, with the dentinal 

 pulp persisting to a late period of life. General form of the skull as in the 

 SciurUUs, but lacking the postorbital processes, and otherwise differing. 



Represented by only a single living species (Castor fiber), whose habitat 

 formerly embraced the greater part of the northern hemisphere. Remains 

 of Castor have been described from the Tertiary deposits of Europe, which 



* See the preceding Monograph. 



