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 rutins and Mus bimaculatus for 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 Eligmodontia, 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, and 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 Reitlirodon and Hplocheilus as 

 worthy of full generic rank, he makes Hesperoinys to include three subgenera, 

 viz., — Calomys, Waterh. ; Hdbrothrix (—Habro/hrix plus Phyllotis, Waterh.) ; 

 and Oxym icterus ( — 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 leucopus 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 Ileithrodon in North 

 America. Holochilus has no nearer representative than Sigmodon, which is 



* Really four subdivisions, as he distinguishes the naked-footed species (culifoniicits and eremious) 

 from the leucopus typo ; 1ml he very properly refrains from naming this Bection. It does not appear 

 tu us to have even subgenera- value, as the barcfootedncss is merely au accident of the animal's desert 

 habitat. 



