194 Mr. O. Thomas on the South- American 
diverse characters of some of the forms included in it. In 
this revision, by removing some of the most diverse into 
special genera, the groups that remain are rendered more 
clear-cut and definable, to the great advantage of students 
of the subject. 
There would appear to be five genera of this group that 
might be recognized, as shown in the following key :— 
A. Molars of normal Oryzomys structure, with 
brachyodont crowns. 
a. Anterior zygomatic plate not projecting for- 
wards enough to be visible from above. 
a’, Without raised supraorbital beading con- 
tinued across brain-case. MM?’ of evenly 
oblong shape, its antero-internal tubercle 
not or little reduced. Mamma 1—2=6. 
a’, Interorbital region flat, its edges forming 
overhanging divergent ledges. Feet 
broad, with flattened pads ; claws short, 
strongly curved, broad at base ........ 1. Rhipidomys. 
b*. Interorbital region without ledges, though 
in the larger species the edges may be 
sharpened. Feet narrow, with high 
pads; claws longer, less curved, nar- 
DOWEL Mak stay 74 xi Wargo bie Oe ecg ee 2. Thomasomys. 
b?, With raised supraorbital beading continued 
across brain-case. MMZ' not evenly oblong, 
the antero-internal cusp reduced. Mamme 
Seuss spate Ce poe eae ath wig ee Ree 3. Phenomys. 
6, Anterior zygomatic plate projecting forwards, 
visible from above. Skull with long heavy 
muzzle and small brain-case.............. 4, Delomys. 
B. Molars of less typical Oryzomys structure, the 
crowns slightly hypsodont. Build of skull 
suggesting Oxymycterus or Microxus........ 5. Inomys. 
The removal of the species ferrugineus, dorsalis, and sub- 
lineatus from Thomasomys simplifies the definition and 
reduces the range of that genus, as these three were out- 
lying Brazilian forms with a different mammary formula, as 
compared with the true Thomasomys, which inhabits the 
northern part of the Andean area, with extension eastwards 
to British Guiana. 
One Andean species also, inmcanus, shows such marked 
special characters that I have formed a peculiar genus for 
its reception. 
The following are short diagnoses of the genera now 
recognized, with lists of the species included in them. 
