480 
U. S. P. R. R. EXP. AND SURVEYS—ZOOLOGY-GENEIiAL REPORT. 
leucopus ; they are thin, membranous, and sparsely clothed with scant hairs. The tail is very 
long, considerably exceeding the body ; it is more finely annulated than in H. leucopus, and 
covered with hairs, which, however, do not conceal the annuli. 
The hair of this species is quite long, though not particularly soft. The upper parts are of a 
light grayish yellow, finely lined by the black tips of the hairs ; the cheeks and lower part of 
sides with rather a distinct stripe of light ochrey yellow. The under parts and the feet are 
pure white ; the tail is uniform in color, or, at most., a little lighter beneath, the line of sepa¬ 
ration entirely inappreciable. 
The colors of this species are more yellow, and much lighter, in every way, than in H. 
leucopus. It is much smaller and lighter colored than H. californicus, in which we have 
similar characters of naked soles, very long and unicolor tail, &c. 
List of specimens. 
HESPEROMYS (ONYCHOMYS) LEUCOGASTER. 
Missouri Mouse. 
Hypudaeus leucogaster, Pkinz Max. Reise in das innere Nord-Amerika, II, 1841, 99. (Fort Clark.) 
Jins Missouriensis, Aud. & Bach. N. Am. Quad, II, 1851, 327 ; pi. c. 
Sr. Ch.—C olor above, grayish brown, passing into yellowish red, and finally into a stripe of fulvous on the sides. Feet, 
including outer surface of the fore arm and under surface of the body and tail, white. For generic characters see page 458. 
This species, in some respects, hears a resemblance to Arvicola, especially in the short tail, 
and rather short ears ; it is in general appearance, however, nearer to the ordinary white-footed 
mice. Still, in many respects, there is a striking .difference from both, to he hereafter 
referred to. 
The head is proportioned much as in Hesperomys leucopus, though with a broader muzzle ; the 
muzzle is entirely hairy, except on the septum; there is rather more hair than in H. leucopus. 
The lip is cleft nearly to the septum. The eyes are large, the whiskers long, and white and 
black. The ears are small, about two-thirds the height, and half the area of H. leucopus. The 
antitragus is low and very slightly valvular, less so than in H. leucopus. The membrane is 
rather thick, the surfaces of the ear well coated with short hairs, longest on the convexity, and 
nearly naked around and anterior to the meatus. 
The most conspicuous feature of this mouse is in the peculiar proportions of the limbs. The 
