648 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
mens from the Black Hills. This form becomes even still paler and more 
fulvous at the eastern base of the main chain of the Rocky Mountains, be- 
tween latitude 43° and 47°, where it begins to pass by insensible stages of 
gradation into the so-called Sciurus richardsoni of the Rocky Mountains 
north of 45°, and the so-called Sciurus fremonti of the Rocky Mountains 
south of about the same parallel. In the collections made in Western 
Wyoming, near the Yellowstone Lake, occur many specimens which are so 
exactly intermediate between the three forms (S. hudsonius, S. richardsoni, 
and S. fremonti), whose habitats here meet, that it is impossible to say which 
of the three they most resemble. At the same time, specimens can be 
selected which will form a series of minute gradations from the pale form of 
hudsonius from the Plains, on the one hand, to the richardsoni and fremonti 
forms on the other. To the southward of this district we soon pass into the 
region of the typical fremont, and to the westward and northward into the 
habitat of the richardsoni type. Even the country about the sources of the 
Gros Ventres Fork of the Snake River is already within the range of the 
true richardsoni.* The habitat of S. richardsoni extends from the main 
chain of the Rocky Mountains, north of latitude 44°, to the Cascade Range. 
Here it becomes mixed with S. douglassi, which scarcely differs from S. 
richardsoni, except in being a little darker above, and in having the ventral 
surface more or less strongly tinged with buff, varying in different specimens 
from cinereous to pure buff. This form prevails from the Cascade Range to 
the Pacific coast, southward to Northern California, and northward probably 
to Sitka. In Northern California, the S. douwglassi meets the range of the 
true S. fremonti, between which two forms there is here the most gradual 
and intimate intergradation. In this group, we have hence four forms, which 
in their extreme phases of mutual divergence, appear as diverse as four good 
congeneric species need to, but which, at points where their respective hab- 
itats join, pass into eich other as gradually as do the physical conditions of 
the localities at which their extreme phases are developed. 
“The Tamias quadrivittatus group} presents an equally, or even more, 
striking range of variation in color, and also varies to an unusual degree in 
“*While the prevailing color above in S. hudsonius is light yellowish-brown, varying to bright fer- 
Tuginous along the middle of the back, in S. richardsoni it is dull rusty or dark chestnut-brown, and in 
S.fremonti pale brownish-gray. The prevailing color of the tail in S. hudsonius is usually yellowish- 
rusty, varying to dark ferruginous, with broad annulations of black ; in S. richardsoni, it is black, varied 
more or less with rusty; in S. fremonti, black, varied with gray.” 
“ft Tamias quadrivillatus, T. pallasi, T. townsendi, and T. dorsalis of American authors.’ 
