338 Mr. O. Thomas on the South- American 
Genotype. Z. cherriei, Allen (Oryzomys cherriez, Allen). 
The recognition of Zygodontomys as a genus distinct from — 
Akodon is somewhat provisional, and can only be finally 
settled when specimens with unworn molars of many more 
species are available for examination. Such species as 
Akodon lenguarum undoubtedly tend to connect the two 
groups, but without a series of young specimens a definite 
conclusion is not easy to arrive at. 
II. Axopon, Meyen. 
Fur generally less coarse than in Zygodontomys. Colour 
variable, the more typical species greyish olivaceous, without 
special markings. Claws rather variable, the members of 
the obscurus group with rather longer fore-claws than is the 
case in the others ; a specimen of A. boliviensis has fourth 
fore-claw 2°0 mm., hind 2°7, of A. obscurus fore 2°7, hind 3°7. 
Skull more lightly built than in Zygodontomys. Inter- 
orbital region fairly narrow, its edges squared more or less 
sharply, and occasionally with slight beading, but never with 
overhanging ledges. Palatal foramina usually narrowed 
behind, penetrating far between the molars. Bullze compara- 
tively small. 
First upper molar with a well-defined groove or valley on 
its anterior surface between the cusps of the first lamina ; 
this wears out as age advances, but slight traces of it may 
generally be perceived. 
T'wo subgenera, as follows :— 
Il. A. AKopov, s. s. 
Fur normal. Colour grey, pale brown, or olivaceous. 
Skull of normal shape; the brain-case and interorbital 
region fairly narrow. 
Genotype. Akodon boliviensis, Meyen. 
II. B. Cuatcomys, subgen. nov. 
Fur fine and velvety. Colour dark brown or coppery, the 
under surface scarcely lighter than the upper. 
Skull with large rounded brain-case and broad interorbital 
region. : 
Genotype. Akodon (Chaleomys) erosus, Thos. 
Other species: wrichi, All., venezuelensis, All., meridensis, 
All. 
This subgenus contains the blackish Akodons, which, like 
Melanomys in relation to Oryzomys, form a special group in 
the tropical countries of north-western South America, | 
