SCIURKIDZ—GEOGRAPHICAL DISTRIBUTION AND VARIATION. 647 
among the Squirrels. Others, however, show the transition that obtains in 
passing from the moist, fertile prairies of the Mississippi Valley to the dry 
plains, or from the deserts and mountainous districts of the interior to the 
moist region bordering the Pacific coast north of the parallel of 40° 
Spermophilus tridecem-lineatus furnishes a good illustration of the differences 
in color that occur between representatives of the same species living on the 
moist, fertile prairies and those inhabiting the dry, barren plains, those from 
Illinois, Wisconsin, Minnesota, and Iowa being much darker than those from 
Western Nebraska, Western Kansas, and Colorado. Even specimens from 
Kastern Kansas are much darker than those from the middle and western 
portions of the same State. In this species, the color is varied, in passing 
from the prairies to the plains, not only by the lighter shade of the dark 
ground-color, but by the considerably greater breadth of the light spots and 
stripes in the specimens from the plains. The Spermophilus grammurus 
group (composed of the S. grammurus, S. beecheyi, S. douglassi, etc., of 
authors) illustrates not only a similar variation in intensity of color between 
the inhabitants of dry and moist regions, but also a somewhat changed style 
of coloration. Beginning with the nearly uniformly gray or grizzled type of 
Texas and Southeastern New Mexico, we pass to the more rufous or reddish 
phase of the central portions of the Rocky Mountains (in Colorado), which 
also has an increased amount of hoariness on the sides of the neck and 
shoulders, to the form west of the Sierra Nevada Mountains, typically repre- 
senting the Spermophilus beecheyi, in which the hoariness forms broad lateral 
hands separated by a narrow brown medial stripe. This form in Northern 
California passes into the so-called Spermophilus douglassi, which differs chiefly 
from S. beechey? in having the medial stripe darker, or nearly black. 
“Two of the most instructive and interesting groups of the Sciuride, in 
this connection, are those of the common Sceurus hudsonius and Tamias 
quadrivittatus, {*] the former ranging over the northern half of the continent, 
and the latter extending over the western half of North America and Eastern 
Asia. In the Sciwrus hudsonius group, we have at the east the well-known 
Chickaree (S. hudsonius), extending westward to the Plains and northwest- 
ward to Alaska, with its brighter and smaller southern form in the Eastern 
Atlantic States. On the arid plains of the Platte and Upper Missouri Rivers, 
it presents a markedly paler or more fulvous phase, well illustrated by speci- 
* Tamias asiaticus of the present memoir. See posied, the account of the genus Tamias. 
