MURIDA!—SIGMODONTES—ONYCHOMYS. 107 
obtuse rostrum. The margins of the orbits are not beaded as in Oryzomys. 
The descending process of the lower jaw is, perhaps, not quite so flat and 
quadrate, being a trifle more bent and angular, slightly foreshadowing the 
hamular process of Arvicoline ; and, likewise, the coronoid is longer, rising 
high above the condyle. To our view, the molars present unmistakably a 
slight deviation from Hesperomys toward Arvicoline in being somewhat pris- 
matic; that is, with sharp salient and reéntrant angles; the borders of the 
molar series being serrate, rather than crenate as in Hesperomys. Never- 
theless, no generic dental characters of Hesperomys are here violated. 
On casually picking up a specimen of Onychomys leucogaster, one 
might be forgiven for supposing the tail to be broken off—this member is so 
short and “stumpy”. The base is very thick, and the tapering to an obtuse 
point is rapid. The tail is well clothed with hairs, as usual; but these only 
furnish an apology for a terminal pencil. Both the absolute and relative 
length of the tail is shown in the table below. The large fore feet, armed 
with remarkably long and little curved claws, point to fossorial powers, and 
possibly to subterranean habits, not shared by any other Hesperomys. Next 
in importance, after the relative sizes of the fore and hind members (which 
latter are unusually short), must be ranked the peculiar condition of the soles, 
in which the two posterior of the six tubercles usually seen both in Hespero- 
mys and in Arvicola have disappeared, or at least cannot be detected beneath 
the fur that densely covers the metatarsus, leaving only four tubercles at the 
bases of the toes. The middle finger is the longest of all; the fourth next; 
the first is rudimentary ; the fifth is between the second and first; the second 
between the fifth and fourth. The inner toe is very short; the outer inter- 
mediate between this and the other three, which are all of about the same 
Iength. The ears, as we have said, are, in the type of the subgenus, much 
smaller than in Hesperomys proper, and very much as in Lvotomys ; they are 
closely pilous both sides. 
The fur is very compact and lustrous, with few long hairs; the whiskers 
are numerous and very long; the eyes are larger than in Arvicolines, but have 
not the size and prominence of those of the true Murines. The upper lip is 
cleft to the nasal papillee, and the entire muffle, except these protuberances, is 
hairy. The bicolor pattern of coloration is exactly as in Vesperimus. 
This section is based upon the Hypudeus leucogaster of Maximilian, after- 
ward the Mus missouriensis of Audubon and Bachman. To the original type- 
