AG MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
to Hesperomys of Baird, as limited by that author, as above cited. Calomys, 
as used by Audubon and Bachman for their Mus aureolus, is not a synonym, 
subgenerically, of Calomys, Waterhouse ; aureolus being strictly of the same 
group as leucopus, while Calomys of Waterhouse refers to the South American 
elegans, bimaculatus, &e. The only other name, except Hesperomys, that we 
can find has been applied to the present subgenus, is Muscudus of Ratinesque. 
This name, in strict technical conformity with the rules of nomenclature, 
ought to be adopted ; and it is nothing to the point that its proposer did not 
properly define it, for we know that he used it in connection with deucopus. 
But the name is so inseparably connected with Mus musculus, that to use it in 
a different connection, aud one where the tribal distinctions from Mus especially 
require to be signalized, would result in an evil of far more consequence than 
the breach of a rule of nomenclature. While we regret that we happen to be 
personally interested in this, one of the rare cases where the law of priority 
must be set aside, nevertheless we cannot disregard its obvious requirements. 
The following paragraph is diagnostic of the subgenus Vesperimus :— 
Cuars.—Of medium and small size, lithe form, and quick movement. 
Eyes large, prominent. Snout pointed. Ears large, rounded, thin, finely 
scant-pilous ; antitragus evident but not valvular. Fore feet hardly: or not 
half as long as the soles; palms naked; fore claws not larger than hind claws, 
that of the obsolete thumb rather a nail; other fore digits slender, 3d and 4th 
subequal and longest, 2d and 5th successively much shorter. Hind feet long, 
slender; soles 6-tuberculate, naked, or scant-furred on the posterior third ; 
2d, 3d, and 4th toes much longest and subequal, 5th shorter, 1st much shorter. 
Tail terete, slender, closely hairy, subequal to the trunk in length (ranging 
from as long as body alone to a little longer than head and body together). 
Pelage soft, close, glossy, with but few longer bristly hairs; feet and under 
parts white or whitish; body and tail more or less distinctly bicolor. No 
lanuginous tufts of hair about the ears. Back upper margin of orbit not 
beaded. 
The skull and teeth of all the North American Hesperomys are so much 
alike, while the external form is so different in the three sections of Vesperi- 
mus, Onychomys, and Oryzomys (as will be evident upon comparing the 
diagnoses given beyond of these genera), that we have preferred to define 
Vesperimus chiefly by external characters. The skull of Onychomys merely 
differs from that of Vespertmus in being a little larger and heavier, not quite 
