RODENTIA —-MURINAE-HESPEROMYS. 
457 
tions) to those of Habrothrix, as does the skull likewise. Indeed, it may be said that the species 
of the group resemble Calomys in external form, and Habrothrix in skull and dentition. 
As already stated, this group is divisible into two, one with the soles hairy behind the 
tubercles, the other with the soles entirely naked. The two are very similar in other respects. 
Although a new sub-generic name might with all propriety be made for this section, I shall 
not attempt it now in my ignorance as to how far some of the smaller and least studied South 
American species may share the same characters. At the same time, however, I shall not be 
at all surprised to find that neither this nor any other section of vesper mice has any direct 
representatives on the continent of South America. 
The second division resembles the Arvicolae somewhat in external characters. The eyes are 
large, the ears rather small; the fore limbs are stout and powerful, compared with the hinder, 
which are much shorter in proportion than in the preceding group. The fore feet are two-thirds 
the length of the hinder ; the claws of all the feet very long, the anterior considerably longest. 
The thumb, however, is rudimentary, with a nail. The tail is less than half the length 
of head and body, very thick at base, and tapering rapidly to an attenuated tip, well covered 
with soft appressed, not bristly, hairs. The soles have but four tubercles, lacking tlmtwo posterior 
ones of the preceding group. The sole is very densely furred to the tubercles, and consequently 
much beyond what is seen in the preceding section, or two-thirds instead of one half. This 
furring, too, is much closer and fuller ; in fact, somewhat resembling that of the hares, with 
almost as long a staple in proportion. The fur is soft; the bristly hairs projecting but slightly. 
The skull presents few differences from the preceding division, except that the muzzle and 
inter-orbital spaces are broader, the incisive foramina shorter and much more open, and the 
teeth rather broader. 
In some respects this group resembles Habrothrix, especially in teeth, but differs in having 
the muzzle broad and short rather than slender and long, as compared with Calomys. The 
long fossorial claws, much longer anteriorly than posteriorly, are not found in Habrothrix, 
which also has the sole nearly or quite naked, with black tubercles, as in Sigmodon. From 
Scapteromys it is distinguished by the thumb nail, not claw, and by the character of the molars. 
The name of Onychomys may be applied to it, as referring to the long claws. There is but one 
species thus far known: the Hypudmus leucogaster, of Maximilian, or the Mus missouriensis of 
Audubon and Bachman. 
The third and last group has Hesperomys palustris as its typical, and, as far as known, its 
only species. Here the characteristics are very strongly marked. The general appearance is 
that of Sigmodon or even of Arvicola, from both of which, the tail considerably longer than head 
and body, will at once distinguish it. The fur, though lustrous, is yet very coarse externally, 
owing to the great predominance of the bristly hairs, which are double the length of the others. 
The hair generally is longer and much coarser than in the other divisions, only less so than in 
Sigmodon. The ears are comparatively small, not longer than the adjacent fur. The hind feet 
are very long, especially the toes, which are inserted obliquely, as in Fiber. The soles are 
perfectly naked, with six tubercles, which are all very small, except the posterior, which is 
narrow, linear, and very long, as in Mus. The skull in most respects, the teeth especially, is 
as in the preceding sections ; a very striking difference, however, is seen in the upper margin 
of the orbit, which is extended into a sharp, compressed crest, passing backwards and sinking 
on the temporal region. This is seen also in Sigmodon, and is the character described as existing 
in the large South American Hesperomys. The posterior margin of the bony palate is consider- 
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