108 MR. 0. THOMAS ON RODENTS FROM PERU. [Jan. 17, 
Mamme 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 
Myoxomys* were founded*. These names must, however, both 
stand as synonyms of Rhipidomys, Tschudi, H. leucodactylus cer- 
tainly belonging to the same subgenus as HH. sumichrasti*. It is 
true that whereas the ordinary S. American Rhipidomyes are dis- 
tinguished from true Hesperomys by having only 6 mamma, 
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 difference is only in degree, and not in kind, so 
that I think we are justified in amalgamating Myctomys with Rhipi- 
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, 
Tomes’, 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 HA. taczanowskii would seem to 
be most nearly allied to Rhipidomys, notwithstanding its rounded 
supraorbital margin and untufted tail, and H. 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 mamme, a number characteristic of the ordinary 
members of that group. 
10. H. (Rureipomys) cINEREUS, sp. un. (Plate IV.) 
a. Cutervo, 9200', February or March 1879. 
Head and Forearm Ear-conch, Muzzle 
body?. Tail. Hind foot. and hand. length, to ear, 
OQ ATS 5:0 1:14 1°38 64 1°18 
Skull-dimensions. 
Back of Breadth of con- 
Total Greatest Molar incisors to striction between Lower jaw 
length. breadth. series, 1st molar. orbits. (bone only), 
tes 8S) 73 22 °37 19 ‘78 
' Rev. et Mag. Zool. 1860, p. 107. 
2 P.Z.8. 1861, p, 284. 
3 See Alston, Biol. Cent.-Am., Mamm. p. 143, 1880. 
4 Mr. Tomes himself, when first describing Myoxomys, placed in it, besides 
H. sumichrasti (its type and most typical species), 4. latimanus, Tomes (=H. leu- 
codactylus), and H, bicolor, Tomes. 
5 See above, p. 101. 
6 P. Z.8. 1860, p. 117. 
* These measurements were taken before the skull was extracted. 
