46 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



(o Hesperomys of Baird, as limited b\ that author, as above cited. Calomys, 

 as used by Audubon and Bach man for their Mus aureolas, is not a synonym, 

 subgenerically, of Calomys, Waterbouse; aureolus being strictly of the same 

 group as leucopus, while Calomys of Waterhouse refers to the South American 

 elegans, bimaculatus, &c. The only other name, except Hesperomys, that we 

 can find has been applied to the present. subgenus, is Musculus of Rafinesque. 

 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 leucopus. 

 But the name is so inseparably connected with Mus musculus, that to use it in 

 a different connection, and 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 : — 

 Chars. — 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 Vesperimus in being a little larger and heavier, not quite 



