466 U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



HESPEROMYS AUSTERUS, Baird. 



Hesperomys austenu, BAIBD, Pr. A. N. Sc. Phila. VII, April, 1855, 336. 



SP. CH. Fur very full and soft ; rather smaller than II. leucopus ; feet larger in proportion. 



Young, dark slaty plumbeous. Adult, almott 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. 



