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 Castortde 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 (Steneofiber, 
including Pal@ocastor Leidy and Chalicomys) differ widely from Castor, while 
one (Castoroides) 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 Steneofiber 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 Séeneofiber 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 Castoride, has, 
as already shown,* only a superficial resemblance to the true Beavers. 
Grnus CASTOR Linn. 
Castor L1nn., Syst. Nat., i, 1766, 178. : 
. Cuars.—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 #4, single-rooted, with the dentinal 
pulp persisting to a late period of life. General form of the skull as in the 
Sciuride, 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. 
