sciuitnxas— 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° 

 SpermophUus 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 

 Eastern 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 SpermophUus grammurus 

 group (composed of the 8. grammurus, S. heechcyi, S. douglassi, etc., of 

 aut hoi's) 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 SpermophUus beecheyi, in which the hoariness forms broad lateral 

 bands separated by a narrow brown medial stripe. This form in Northern 

 California passes into the so-called SpermophUus douglassi, which differs chiefly 

 from S. beecheyi in having the medial stripe darker, or nearly black. 



" Two of the most instructive and interesting groups of the Sciurida, in 

 this connection, are those of the common Sciurus 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 Sciurus 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 asialicus of the vireseut memoir. See posted,, the account of the geuus Tamias. 



