108 MR. O. THOMAS ON RODENTS FROM PERU. [Jan. 17, 



Mammae 4 or 6. Skull with more or less well-marked supraorbital 

 ridges. 



The most peculiar member of the subgenus is H. sumichrasti, 

 De Sauss.', upon which both that author's Nyctomys and Tomes's 

 Jlyoxomys - were founded '\ These names must, however, both 

 stand as synonyms of Rhipidomys, Tschudi, H. leucodactylus cer- 

 tainlv belonging to the same subgenus as H. sumichrasti ^. It is 

 true that wliereas the ordinary S. Kmmcd^n Rhipidomyes are dis- 

 tinguished from true Hesperomys by having only G mammae, 

 H. sumichrasti outdoes them all in this respect by having only 4% 

 and in other ways is the most markedly specialized of them all ; but 

 nevertheless the diiference is only in degree, and not in kind, so 

 that I think we are justified in amalgamating Nyctomys with Ehipi- 

 domys as but one subgenus. 



There are, however, in addition to the well-marked species already 

 referred to, three at least which, while they possess in different 

 degrees some of the essential characters of Rhipidomys, yet are 

 more or less intermediate between the true Hesperomys and the most 

 typical members of this subgenus. The first of these is H. bicolor, 

 TomesS the position of which, however, I cannot properly determine, 

 not having seen a specimen. The other two are those next follow- 

 ing, both new to science, of which H. tdczanovJsHi would seem to 

 be most nearly allied to Rhipidomys, notwithstanding its rounded 

 supraorbital margin and untufted tail, and JI. cinereus the least, 

 having, in addition to these two last-mentioned characters, feet pro- 

 portionally longer, with less Rhipidomyine foot-pads, and a more or 

 less bicolor tail. Notwithstanding these differences, however, I 

 prefer for the present to call these both Rhipidomys, as they have 

 only three pairs of mammse, a number characteristic of the ordinary 

 members of that group. 



10. H. (Rhipidomys) cinereus, sp. u. (Plate IV.) 

 a. Cutervo, 9200', February or March 1879. 



Head and Forearm Ear-couch, Miizzle 



a. 



• Rev. et Mag. Zool. 1860, p. 107. 

 2 P. Z. S. 1861, p. 284. 



^ See Alston, Biol. Cent. -Am., Mamm. p. 143, 1880. 



* Mr. Tomes himself, when first describing Myoxomys, placed in it, besides 

 H. sumichi-asfi (its type and most typical species), H, latimanus, Tomes (=/f. leu- 

 codactylus), and H. bicolor, Tomes. 



5 See above, p. 101. 



6 P. Z. S. 1860, p. 117. 



" These measurements were taken before the skiill was extracted, 



