> 6) 
Muridse commonly referred to Akodon, 337 
The following key to the genera may assist in the deter- 
mination of specimens :— 
A. Supraorbital edges more or less squared. 
a. Claws normal. Bulle not enlarged. 
a”, Form unmodified; tail of medium length; 
eyes not reduced. 
a*. Supraorbital edges square or beaded, 
but without overhanging ledges. 
a‘, Supraorbital edge beaded. .M' with- 
out notch on anterior face........ I. Zygodontomys. 
6*, Supraorbital edge not or scarcely 
beaded. MM? with anterior notch.. Il. Akodon. 
@. Fur normal. Colour grey, oliva- 
ceous, or pale brown .........- II. A. Akodon, s. 8. 
6°. Fur velvety. Colour dark brown 
OLCOPPELY.” “a's. - 1-1 ad cee ees .. IL. B. Chalcomys. 
b°. Supraorbital edges with overhanging 
ledges. MM?! with notch............ Ill. Thalpomys. 
&. Form Pitymys-like, with short tail and 
reduced eyes. M* with notch........ IV. Thaptomys. 
b. Claws elongated. Interorbital region broad, 
with squared edges. Bullze enlarged. M* 
Aye OMG OCE IT.) celal ae loiciay sie widely « sla els V. Belomys. 
B. Supraorbital edges rounded. MM’ without an- 
terior notch. 
a, Snout not elongated. Brightly contrasted 
markings present. J ore-claws lengthened. 
Balle darcy bie able 2 as ee « erie Goce : VI. Chreeomys. 
6, Snout elongated. Colorationnormal. Fore- 
claws not lengthened, Bulle normal.... VII. Adrothriz. 
JI. Zyaovontomys, Allen. 
Form normal. Fur coarse. Colour coarsely lined brown, 
the lining heavier than in any other members of the group. 
Claws of normal proportions, the fourth * fore-claw 2°1 mm. 
in a specimen with hind foot 24 mm., and the fourth hind 
claw 2°6 mm. 
Skull comparatively strongly built, its upper outline evenly 
bowed. Supraorbital edges square, with well-defined beads 
running back across the parietals. Zygomatic plate well 
thrown forward, its front edge often slightly concave. 
Palatal foramina scarcely or not narrowed behind, and not 
penetrating so far between the molars as in most of the 
succeeding genera. 
First molar without a deep groove separating the two 
cusps of the anterior lamina. 
* Although the third claw is generally a little longer than the fourth, 
the two vary together, and there seems no objection to taking the length 
of the fourth as being by far the most convenient to measure on dried 
skins, the third being commonly hidden behind the fourth. In measuring 
the corresponding claw behind, care must be taken to remember the 
reversed position of the foot in modern skins. 
