108 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
species we have had the pleasure of adding a new and interesting variety, if not 
species, from a hitherto unsuspected locality. The following are the compar- 
ative diagnoses of the two forms :— 
Beneath, snow-white ; above, mouse-brown, with darker 
dorsal area Tail twice the hind foot or less; much 
less than half the head and body. Fore foot more 
than half the hind foot. Ear about 0.50 high. ----- LEUCOGASTER. 
Beneath, tawny-white;* above, brownish-fulvous, with no 
darker dorsal area. Tail about two and a half times 
the hind foot; almost half as long as the head and body. 
Fore foot half the hind foot. Ear about 0.75 high. - - - (var.?) ToRRIDUS 
HESPEROMYS (ONYCHOMYS) LEUCOGASTER (Maxim.). 
Missouri Mole-Mouse. 
Hypudaus leucogaster, MAXIMILIAN, Reise in das Innere N.-Amer. ii, 1841, 99 (Fort Clark). 
Hesperomys (Onychomys) leucogaster, Barrp, M. N. A. 1857, 480 (Nebraska).—Cougs, Proc. Acad. Nat. Sci. 
Phila. 1874, 182. 
Hesperomys lcucogaster, MAXIMILIAN, Arch. Naturg. xviii, 1862, pl. 4, f. 8; Verz. N.-A. Siiug. 1862, 161, fig. 
Mus missouriensis, AUD. & Bacu., Q. N. A. ii, 1851, 327, pl. 100 (Fort Union). 
Hasirat.—Upper Missouri region. Red River of the North (Cowes). 
Our notice of the subgenus Onychomys has necessarily involved the 
details of its type-species so largely that but little remains to be added. The 
absolute dimensions and relative proportions of several specimens, most of 
them additional to any heretofore published, are fully elucidated in the 
following table. 
In color, the animal closely resembles Hesperomys leucopus—so closely 
that it is difficult to describe any differences. When young, it is of the same 
plumbeous-gray above and ashy-white beneath; and, with advancing age, 
these colors change to snowy-white below and fulvous-brown above, with a 
dusky dorsal area. The feet are white; the tailis bicolor; the whiskers are 
black, with a few white ones intermixed. The chief distinguishing feature 
in coloration, as compared with Hesperomys leucopus, is the mostly white 
muzzle. In leucopus, the white usually slopes down to the lips, leaving the 
nose of the color of the forehead; while, in /eucoguster, the white keeps on, 
and embraces the snout. This curious difference holds good in all the speci- 
* As intimated beyond, we are not perfectly sure that the yellowish tint is not due to discolora- 
tion in alcohol. 
