Colorado Red-Backed Mouse 103 
in burrows. Though mainly nocturnal they are sometimes 
seen in daytime. They eat seeds of any kind, also grass. 
They do not hibernate, and Bailey says that they do not lay 
up any stores of food for winter use. 
From four to six young are born in a single litter, and the 
breeding season seems to be from May into July at least, 
for very small young were taken at Mud Springs, Garfield 
County, in the middle of that month. 
Genus PHENACOMYS (Grk. phcnax, an impostor, false, + mus, 
mouse, false mouse, in allusion to the fact that, deceived by 
external appearances, naturalists long classified the species 
of the genus with MicroUts). 
Phenacomys Merriam, A^. A. Fauna, No. 2, p. 28 (1889). Type 
P. intermedins . 
Revision, Miller, Proc. Biol. Soc. Wash., xi., p. 8 (1897). 
Medium or small-sized voles with short rounded tails less than 
half the total length, and with the soles of the fore feet with 5 and 
of the hind feet with 6 tubercles; dentition, i. ^; m. f X 2 = 16; 
lower incisors with short roots ending posteriorly at about the level 
of the molar teeth; molars distinctly two-rooted in the adult, the 
pattern of the upper ones showing a series of reentrant angles 
approximately equal in depth on both the inner and outer sides, 
while in the lower molars the angles on the outer side are far deeper 
than those on the inner. 
Some nine or ten species of this genus have been described, 
they are chiefly confined to the boreal regions of North Amer- 
ica extending as far south as the Rocky Mountains of 
Colorado while one species has been found in California. 
Two species are recorded from Colorado. 
Phenacomys orophilus (Grk. oros, mountain, + philein, 
to love). Idaho Mountain Vole. 
Phenacomys orophilus Merriam, A'. A. Fauna, No. 5, p. 65 (1891). 
Key of the Species 
Color gray. 
Color decidedly ochraceous. 
P. orophilus, p. 103. 
P. preblei, p. 104. 
