Allen.] 280 [February 4, 



Douglassij which differs chiefly from S. Beeclieyi in having the medial 

 stripe darker, or nearly black. 



But two of the most instructive and interesting groups of the 

 Sciuridoz, in this connection, are those of which the common Sciurus 

 hudsonius and Tamias quadrivitalus are respectively familiar exam- 

 ples, 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 northwestward 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 specimens 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, between lat- 

 itude 43° and 47°, where it begins to pass by insensible stages of 

 gradation into the so-called Sciurus Richardsoni of the Rocky Moun- 

 tains 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. hudso- 

 nius, S. Richardsoni and S. Fremonti) whose habitats here meet, that 

 it is impossible to say which of the three forms 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 Fremonti, 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. 1 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 



1 While the prevailing color above in S. hudsonius is light yellowish-brown, vary- 

 ing to bright ferrugineous 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. 



