152 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
as long as head to nearly half as long as trunk. Pelage thick, soft, of ordinary 
fur, interspersed dorsally with longer bristly hairs; the fur at the roots uniformly 
plumbeous, colored only at the tips; general colors subdued, the shades 
diffuse, rarely with distinct parti-coloration, but under parts ordinarily lighter 
than upper, and tail bicolor from sharing the respective shades. 
The North American species fall naturally into four groups, as originally 
pointed out by Professor Baird. These groups may be thus conveniently 
analyzed :— 
(a) Back upper molar with 2 external triangles and a posterior 
CTESCENbe ee ate eee eee eae Myonomes. 
1 external triangle and a posterior 
trefoil (0). 
(6) Front lower molar with 3 internal and 2 or 3 external closed 
irinOGlesteer ie cee en Chilotus. 
2 internal and 1 external closed tri- 
angle (c). 
(c) Fore claws not larger than hind claws.....--.-.----- Pedomys. 
obviously larger than hind claws.....:-.-. Pitymys. 
There are no tangible differences in the skull and teeth between Pedomys 
and Pitymys ; nevertheless, the external peculiarities of Pitymys are greater 
than those of either of the other forms. Chd/otus, in general external char- 
acters, is so like both AZyonomes and Pedomys that we cannot distinguish them, 
except by the peculiar construction of the ear. It shares the posterior trefoil 
of the back upper molar with Pedomys and Pitymys, but shares the greater 
number of lateral closed triangles of the front lower molar of Afyonomes. 
Myonomes, again, is not separated from Pedomys by any external character, but 
by its posterior crescent and two external lateral triangles of the back upper 
molar. So, combining dental and external characters, we may draw up the 
following antithetical diagnoses, making combinations perfectly tangible and 
satisfactory :— 
A.—Myonomes. Back upper molar with 2 external triangles and a posterior 
crescent. Middle upper molar with 2 internal triangles. Front lower 
molar with 3 internal and 2 or 3 external lateral triangles. Ear unrim- 
med in front. Sole 6-tuberculate. Fore claws not longer than hinder 
ones. Tail about § the head and body, or more. Pelage ordinary. Of 
maximum and medium size. 
