124 The Mammals of Colorado 
distinct enough to form a lateral line; tail rather indistinctly 
bicolor, blackish above, whitish below, and very scantily haired. 
This description is made from a specimen in the Warren Collection, 
taken at Crestone. 
Distribution. — This species has only been taken in the San Luis 
Valley, and as only the type specimen was known for over fifty years, 
and its exact locality was unknown, not much can be said about 
its range . Gary has recently taken it at the Medano Ranch, near the 
San Luis Lakes, Costilla County, and in October, 1909, I took one at 
Crestone, Saguache County, and have had for some time fragments 
of one from the same place. 
Habits. — These are no doubt similar to those of the other 
species. The Crestone specimen was taken among weeds 
at the edge of a field, and at the same place Deer -mice and 
House Mice were taken. This specimen contained five 
good-sized embryos. 
Genus ONYCHOMYS (Grk. onux, a nail or claw, -f nius, mouse) 
Onychomys Baird, A^. Amer. Mamm., p. 458 (1857). Type O. 
leiicogaster . 
Small, rather thick-set rodents with a very short tail always 
less than half the length of the body; ears short and rather hairy; 
claws, especially those of the fore feet, long and strong; soles of 
the hind feet with the proximal two thirds thickly furred and with 
only the four anterior small tubercles, the larger posterior ones 
wanting; dentition, i.|;m. | X 2 = 16; both upper and lower 
molars with a double row of cusps only. 
About ten species of this genus have been described, 
all from west of the Mississippi \'alley, and extending south- 
wards into Mexico; only two species are found in Colorado. 
Key of the Species 
A. Color more tawny above, upper surface of tail with black hairs. 
O. leucogaster pallescens, p. 125 
B. Back grayish, sides and flanks tawny; upper surface of tail 
like the sides. O. leucogaster brevicaudus, p. 126 
