44 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
ing his comparisons between Mus and the new group he was about to estab- 
lish, he happened to select Mus rattus and Mus bimaculatus tor that purpose. 
We may therefore, with entire propriety, elect Mus bimaculatus as technically 
the type of Hesperomys. When, in 1837, Waterhouse established the sub- 
genus Calomys upon C. elegans, he included in it both bimaculatus and graci- 
lipes. Eligmodontia of F. Cuvier, of the same date, has the same strictly 
consubgeneric species as its type. It is a question, therefore, whether either 
Calomys or Eligmodontia ought not to take precedence over Hesperomys ; but 
as the latter name has become firmly established, and as the prior name 
Calomys is by the same author, and at least as early as Eigmodontia, there is 
really no necessity for a change. 
Resting, then, upon this application of Hesperomys, in its strictest subgen- 
eric sense, to such species as bimaculatus, elegans, ad gracilipes, we will inquire 
how far the name may be extended in its generic application. In his able 
essay of 1857, Professor Baird points out, in elaborate detail, the characters 
of the South American species, and, excluding Reithrodon and Holocheilus as 
worthy of full generic rank, he makes Hesperomys to include three subgenera, 
viz.,—Calomys, Waterh.; Habrothrix (= Habrothrix plus Phyllotis, Waterh.) ; 
and Oxymicterus (= Oxymicterus plus Scapteromys, Waterh.). Recurring to 
the North American forms, he establishes three subdivisions,—Hesperomys, 
Onychomys, and Oryzomys.* We are able to confirm the validity of these 
groups in the most unequivocal manner; the only question being whether 
the deucopus group that Professor Baird left in the subgenus Hesperomys is 
not entitled to subgeneric ‘distinction from the South American bimaculatus. 
Professor Baird himself suggests that this ought to be done; and, so far as we 
can judge from the descriptions of authors, the suggestion is available. 
The Vesper mice of North America, collectively, seem to be differentiated 
from those of South America by characters only less important than that one 
which trenchantly divides them both from Old World Mures. Neotoma of 
North America has nothing whatever to correspond in South America. The 
large, leporine, grooved-incisor species of South America are generically differ- 
ent from the little murine species that have been called Reithrodon in North 
America. Holochilus has no nearer representative than Sigmodon, which is 
* Really four subdivisions, as he distinguishes the naked-footed species (californicus and eremicus) 
from the leucopus type; but he very properly refrains from naming this section. It does not appear 
to us to have even subgeneric value, as the barefuotedness is merely an accident of the animal’s desert 
habitat. 
