192 Mr. O. Thomas on the South- American 
XIX.—On the Arrangement of the South American Rats allied 
to Oryzomys and Rhipidomys. By Orprietp THomas. 
(Published by permission of the Trustees of the British Museum.) 
WHEN writing some years ago * on the relation of Oryzomys 
and Rhipidomys to each other and the allied forms 7homas- 
omys and Cicomys, with lists of the forms belonging to each, 
I indicated certain species as of doubtful position, and I 
have now had anopportunity to re-examine these in the light 
of further material. 
As then explained, all these rats are divisible into two 
groups, those with the structure of the palate as in Oryzomys 
(Oryzomys and Gicomys) and those with it as described and 
figured by Bangs + in his “ Erioryzomys” (Rhipidomys and 
Thomasomys). Of the first of these groups only one species, 
“ Hesneromys’’ rufescens, was referred to as doubtful, and 
of this animal an additional example, without locality, has 
lately been found among some old specimens put aside as 
duplicates, and I have therefore been able to make a further 
study of it. . 
The second specimen, ‘although its skull is very dilapidated, 
happens to show the posterior palate, and is also much 
younger than the type, with almost unworn teeth, so that 
an opinion can be formed as to its systematic position. 
On studying the characters observable on the two 
specimens I find that the animal, while belonging to the 
Oryzomys—Cicomys series, certainly represents a genus dis- 
tinct from any of its allies, its molar structure being indeed 
quite unique in the group. 
It may be called :— 
Ruacomys, gen. nov. 
General facies about asin Gicomys. Feet modified for 
an arboreal life, with large plantar and digital pads. Mamme 
apparently 1—2=6, as in Rhipidomys. 
Skull broad and low, with broad, smooth, rounded brain- 
case; supraorbital edges square, not ridged. Zygomatic 
* Ann, Mag. N. H. (7) xviii. p. 442 (1906). 
+ P. New Engl. Zool. Club, i. p. 96, pl. i. fig. 3 (1900). 
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