194 Mr. 0. 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 



bracliyodont 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. M^ of evenly 

 oblong shape, its autero-internal tubercle 

 not or little reduced. Mammae 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. 



h^. Interorbital region without ledges, though 

 in the larger species the edges may be 

 sharpened. Feet narrow, with high 

 pads ; claws longer, less curved, nar- 

 rower 2. Thomasomys. 



b^. With raised supraorbital beading continued 

 across brain-case. M^ not evenly oblong, 

 the antero-internal cusp reduced. Mammae 



2 —2 = 8 3. Phcenomys. 



h. 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 fe7Tugmeus, 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 difiierent mammary formula, as 

 compared wdth the true Thomasomys, which inhabits the 

 northern part of the Andean area, with extension eastwards 

 to British Guiana, 



One Andean species also, incanus, 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. 



