I40 
The Mammals of Colorado 
found out in the fields away from the houses. In fact it 
seems able to adapt itself to any circumstances, and to find 
a living anywhere. 
In the newly settled districts it soon drives the native 
deer -mice away from the houses into which they have in- 
truded, or, if it does not actually drive them away, they leave 
anyway, and leave the field clear for their Old World 
competitors. 
It is a prolific breeder, six to eight young being born in a 
litter, which soon grow to maturity, and begin bearing young 
themselves at an early age. 
Family CASTORIDiE 
Rodents of comparatively large size and natatorial habits, 
with a broad, horizontally flattened tail and webbed hind 
feet; skull massive, w^ithout postorbital processes, and with the 
angle of the mandible rounded; cheek teeth rootless with re- 
entering enamel folds, two outer and one inner; premolars y. 
This family contains only one genus with two species 
found in the northern portions of the Old and New Worlds. 
Genus CASTOR (Lat., a beaver) 
Castor Linn., Syst. Nat., loth ed., i., p. 58 (1758), type C. fiber. 
Revision, Rhoads, Trans. Am. Philos. Soc, xix., pp. 417-439 
(1898). 
Dentition, i. |; pm. |; m. f X 2 = 20 ; incisors large and powerful, 
the lower pair the longer, with chisel-like edges, and colored deep 
orange-red externally. 
The Beavers which are included in this genus are found 
throughout North America and Europe as far east as the 
Ural Mountains. Only one species, and that differing 
but slightly from the European form, is found in North 
America, but it has recently been split up into several 
geographical races, one of which inhabits Colorado. 
