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U. S. P. R. R. EXP. AND SURVEYS—ZQOX.OGY—GENERAL REPORT. 
ably posterior to the last molar, (twice its length,) and there is quite a deep fossa between the 
corners of the palatine notch and the posterior molar. In our other Hesperomys the palatine 
notch extends as far as the line of the molars, and the fossa just mentioned does not exist. In the 
skull of the South American Hesperomys referred to, the notch is posterior to the molars, as in 
the present group. For this group, with its many distinctive features, I would propose the 
name of Oryzomys. 
I will now sum up, in brief terms, the principal characteristics of these different groups, and 
then proceed to the discussion of the several species. 
Hesperomys, Waterhouse.—Form mouse-like. Tail not less than the body without the head, 
sometimes longer. Claws weak ; hind legs and feet long, the latter with six large conical 
tubercles ; soles naked, or less than half hairy. Upper margin of the orbit sharp, but not 
extended into a crest. 
Onychomys, Baird.—Form arvicoline. Tail less than half the head and body. Claws very 
large, fossorial, the anterior longest. Soles with only four tubercles, the two posterior of the 
other groups wanting ; the posterior two-thirds of the soles densely furred. Skull without 
orbital crest; the upper margin of the orbit sharp. 
Oryzomys, Baird.—Form rat-like. Ears nearly buried in the fur. Hairs of body coarse. 
Tail longer than head and body ; the hairs longest on the under surface. Hind feet very long. 
Soles naked, with six tubercles, all very small except the posterior, which is very long and 
narrow. Upper margin of the orbit raised into a compressed crest, as in Sigmodon. 
The following synopsis may serve to show the relationship of the North American species of 
Hesperomys as restricted above. In all, except when otherwise stated, the belly and feet are 
of a more or less pure white. 
A. —Soles hairy behind the tubercles. 
a .—Tail nearly, or about as long as the head and body ; hind feet long, exceeding .15. 
1. Tail distinctly bicolor. 
1. Above yellowish brown, with a broad well defined wash of darker along 
the back. Ears dusky brown, with the extreme edge only hoary. 
2. Above reddish brown, with darker hairs, but no distinct dorsal stripe. 
Outside of fore leg like the back ? Ears smaller than in the preceding, 
dusky, without any hoary edging ?. 
3. Above yellowish brown, with dark hairs, but no distinct dorsal stripe. 
Outside of fore leg white ? Ears very large. 
4. Fur very soft and full. Above sooty brown, relieved, especially on the 
sides, by a little yellowish brown. Outside of fore leg dusky. 
5. Color light cinnamon red ; a tinge of the same on the belly. Ears dusky 
cinnamon... 
leucopus. 
texanus. 
austerus. 
nuttalli. 
. 2. Tail scarcely lighter beneath (shorter than in section 1.) 
6. Larger than H. leucopus. Above rusty yellow, with a dark dorsal band. 
Hind foot .90. gossypinus. 
7. Small® - than II. leucopus. Above yellowish brown, with a dusky dorsal 
band. Ear dusky, without any light edge. Hind foot about .85. cognates. 
