466 U. Ss. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
HESPEROMYS AUSTERUS, Baird. 
Hesperomys austerus, Batrp, Pr. A. N. Sc. Phila. VII, April, 1855, 336. 
Sp. Cu.—Fur very full and soft; rather smaller than ZZ. leucopus; feet larger ip proportion. 
Young, dark slaty plumbeous. Adult, almost sooty brown, slightly pervaded by yellowish brown on the cheeks and 
lower part of sides, the dusky of the sides extending, even in the adult, to the wrist; both feet above and under parts 
white; tail well haired, as long as head and body, lower half white. 
This species is of rather small size and of darker color than any of the North American 
species, except H. michiganensis, from which it may always be distinguished by the longer tail 
and hind feet. 
There are no distinguishing peculiarities in the character of the head. The feet are large, 
the tubercles of the soles also larger than in H, leucopus, the posterior one elongated ; the soles 
are hairy from the heel to the posterior tubercle, the anterior end of which is just half way be- 
tween the heel and the ends of the balls of the toes. The thumb is not so rudimentary as in 
the H. leucopus, and covered by a pretty large nail. 
As far as can be judged, the tail is rather shorter than the head and body, not very stout at 
the base. 
The fur of this species is very dense and full, more so than in almost any other species. The 
prevailing color above is a smoky brown, with a faint grayish yellow tinge on the cheeks and 
sides, rather more vivid on the former. The colors of the back and sides, however, extend 
down to the very wrist. The feet and under half of the tail, with the under parts generally, 
are quite pure white. The tail is well coated with hairs, so as to conceal the annuli, The 
ears are large, brown, with the edges grayish white, in decided contrast. 
The young are of a nearly uniform dark slaty plumbeous above and on the sides, without a 
dorsal stripe of darker. This color also extends plainly down to the wrist. 
As already stated, the longer feet and tail distinguish this species from H. michiganensis of 
Illinois. Neither is there any dusky spot on either fore or hind feet. The dark colors of the 
back are mixed uniformly, shading imperceptibly on the sides, instead of forming a distinct 
broad dorsal stripe. 
Compared with the Upper Missouri species the color is much darker and the tail con- 
siderably longer. Hesperomys leucopus is much larger and has not the sooty tinge; its tail, 
also, is rather shorter. 
Two specimens of Hesperomys (2577, 2578) in alcohol, from Steilacoom, exhibit some quite 
peculiar features among the genus, and very probably constitute an undescribed species. I shall 
not, however, venture to name them, as the material is not sufficient for a full and satisfactory 
description. They have much the appearance of the domestic mouse, but are really Hesperomys. 
A striking peculiarity is seen in the extreme brevity of the hind foot, which in 2578 measures 
but .70 of an inch. The tubercles are all much larger than in the other Hesperomys ; the feet 
broader ; the digits shorter and more nearly equal. The sole is densely hairy to the first 
tubercle. The tail is about as long as head and body ; scantily haired, without concealing the 
annuli, and but little lighter below than above; the colors in very slight contrast. The back 
appears to be dark brownish, the belly and feet yellowish white. One of the specimens is a 
suckling female. . 
The Hesperomys austerus has thus far only been found in the eastern part of Washington 
Territory. 
