458 U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY 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 

 Sesperomys 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 .75. 



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 leucopus. 



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? texanus. 



3. Above yellowish brown, with dark hairs, but no distinct dorsal stripe. 



Outside of fore leg white? Ears very large gamldii. 



4. Fur very soft and full. Above sooty brown, relieved, especially on the 



sides, by a little yellowish brown. Outside of fore leg dusky austerus. 



5. Color light cinnamon red ; a tinge of the same on the belly. Ears dusky 



cinnamon 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. Smaller than E. leucopus. Above yellowish brown, with a dusky dorsal 



band. Ear dusky, without any light edge. Hind foot about .85 coynatiis. 



