MDltlDAE— SIGMODONTES— HESPEROMYS. 45 



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 Habrothrix 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 leucopus 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 Vesp>erimus, 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 Reithiiopon, Waterh. Form leporine. 



b. With smooth upper incisors. 



Subgenus Vesperimus, Coues. Form murine. 

 Subgenus Onychomys, Baird. Form arvicoline. 



Subgenus Oryzomys, Baird. Form rat-like. 

 Genus Holochilus, Braudt. Form rat-like. 



Subgenus Calomys, Waterh. Form murine. 

 Subgenus Habrothkix, Waterh. Form arvicoline. 

 Subgenus Oxy'Micterus, Waterh. 



Geuus Sigmodon, Say. Form arvicoline. 

 Genus Neotoma, Say. Form rat-like. 



Subgenus VESPERIMUS, Coues. 



=zMusenhis, Rafinesque, Am. Monthly Mag. iii, 1318, 440 (leucopus). 



< Hesperomys, of North American writers. 



= Hesperomys, Baird, M. N. A. 1657, 458 (leucopus). 



> Calomys, Aid. i. Bach., Q. N. A. ii, 1851, 303 (aureolus). Not of Waterhouse. 



= Vesperimus, Coues, Proc. Acad. Nat. Sci. Phila. 1874, 178 (type, Mus leucopus, auc 1 .). 



We restrict our term Vesperimus to leucopus and its immediate North 

 American allies. As intimated in the foregoing article, it is exactly equivalent 



