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 leucogastee. 



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.?) tokkidus 



HESPEROMYS (ONYCHOMYS) LEUCOGASTER (Maxim.). 



Missouri Mole-Mouse. 



HypudcBUS leueogaster, Maximilian, Reiso in das Inuere N.-Amer. ii, 1841, 99 (Fort Clark). 



Hesperomys (Onyehomys) leueogaster, Baird, M. N. A. 1857, 480 (Nebraska). — Coues, Proc. Acad. Nat. Sci. 



Pliila. 1874, 182. 

 Hesperomys leueogaster, Maximilian, Arch. Naturg. xviii, 1802, pi. 4, f. 8; Vcrz. N.-A. Sang. 1802, 101, fig. 

 Mus missourieims, Aud. & Bach., Q. N. A. ii, 1851, 327, pi. 100 (Fort Union). 



Habitat. — Upper Missouri region. Red River of the North (Coues). 



Our notice of the subgenus Onyehomys 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 tail is 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 leueogaster, the while keeps on, 

 and embraces the snout. This curious difference holds good in all the speci- 



" As intimated boyoud, wo are uot perfectly sure that the yellowish tiut is not duo to discolora- 

 tion in alcohol. 



