1910] Taylor—Two New Rodents from Nevada. 289 
Creek, 4; Virgin Valley, 1; these localities are in northern Hum- 
boldt County, Nevada, and vary in altitude from 5000 to 9000 
feet. All but the last mentioned are in the Pine Forest Moun- 
tains. 
Callospermophilus trepidus is one of the most common mam- 
mals in the mountains. Ranging from a few hundred feet above 
the desert to the highest point of the mountains, it obviously 
has an extensive Life Zone habitat, being found at least from 
high Upper Sonoran through Transition and Canadian into Hud- 
sonian. 
So far as known trepidus is isolated on this range of moun- 
tains and neighboring spurs. Seeregation at some time in the 
past as a consequence of some environmental change was prob- 
ably one factor in the evolution of the species. 
During the period when local glaciation took place in the 
northern Cordilleran region, the life-zones must of necessity 
have been driven to lower levels. At the close of the glacial 
period the zones probably returned to their former positions. 
As this return process took place, some of the animal species 
now found in this region must have extended their range higher 
and higher, and may be assumed to have become gradually accus- 
tomed to life in their new habitats. This adaptation would 
presuppose the appearance of new characters. Certain ones of 
these species through this adaptation to a different environment 
may have lost all intercourse with others of their own kind. 
This would allow free play to isolation as a factor in evolution, 
and any new characters, whether arising through natural selec- 
tion, mutation or any other cause would tend to be perpetuated. 
Neotoma nevadensis, new species. Nevada Wood Rat. 
Typr.—Adult female; no. 8282, Univ. Calif. Mus. Vert. Zool. ; 
Virgin Valley, Humboldt Co., Nevada, altitude 4800 ft.; May 
17, 1909; collected by Annie M. Alexander; original number 23. 
DixGnosric CHARACTERS.—Similar in general appearance to 
Neotoma desertorum in comparable pelage, but averaging darker 
dorsally; upper surface of tail black; measurements averaging 
less. Skull smaller; incisors narrower and shorter; molars 
