MURIDAN—SIGMODONTES—HESPEROMYS. A5 
an entirely different genus. And although other species of the large genus 
Hesperomys may not necessarily be separated generically, yet the subgenera 
seem entirely different. The form Oryzomys appears not to be represented 
at all in South America ; while, likewise, the form Oxymicterus of South 
America has no exact analogue. Onychomys and Habrothrizx would seem to 
represent each other in their arvicoline form, yet they are abundantly worthy 
of subgeneric separation. Probably the closest approach to identity of form 
is found in the North American /eucopus group (and especially the species 
aureolus or nuttallii, where alone we have yellowness underneath) with the 
Calomys group (i. e., Hesperomys sensu strictiss.); but even here there is 
room for the subgeneric separation that we make in the succeeding article. 
In our comparative ignorance of the South American forms, we shall not 
venture upon any diagnosis or description of the full genus Hesperomys. 
Ample details of the North American forms will be found under the respect- 
ive heads of Vesperimus, Onychomys, and Oryzomys. For similar reasons, we 
refrain from any but North American citations in the foregoing list of 
synonyms. 
We append a table, which may serve to exhibit the correspondences or 
other relations of the North American with South American forms. 
NortH AMERICAN. | SouTH AMERICAN. 
a. With grooved upper incisors. 
Genus OCHETODON, Coues. Form murine. | Genus Rerrnropon, Waterh. Form leporine. 
b. With smooth upper incisors. 
Subgenus VESPERIMus, Coues. Form murine. | Subgenus Catomys, Waterh. Form murine. 
Subgenus ONycHomys, Baird. Form arvicoline. Subgenus Hasrorurix, Waterh. Form arvicoline. 
? Subgenus OxyMIcTEeRus, Waterh. 
Subgenus Oryzomys, Baird. Form rat-like. ? 
Genus Horocuitus, Brandt. Form rat-like. Genus SIGMODON, Say. Form arvicoline. 
? Genus Neoroma, Say. Form rat-like. 
Suscenus VESPERIMUS, Coues. 
= Musculus, RAFINESQUE, Am. Mouthly Mag. iii, 1818, 446 (leucopus). 
< Hesperomys, of North American writers. 
= Hesperomys, Barrp, M. N. A. 1857, 458 (leucopus). 
> Calomys, Atv. & Bacu., Q. N. A. ii, 1851, 803 (aureolus). Not of Waterhouse. 
= Vesperimus, COuES, Proc, Acad. Nat. Sci. Phila. 1874, 178 (type, Mus leucopus, auc*.). 
We restrict our term Vesperimus to leucopus and its immediate North 
American allies. As intimated in the foregoing article, it is exactly equivalent 
