|',4S MONOGRAPHS or NORTH AMERICAN RODENTIA. 



mens from the Black Hills. This form l)ccomcs even still paler and more 

 fulvous ;i( the eastern base of the main chain of the Rocky Mountains, be- 

 tween latitude 4o" 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 t he 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 fremonti, and to the westward and northward into the 

 habitat of the richardsoni type. Even t he 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 bntf, 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. douglassi 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 etch other as gradually as do the physical conditions of 

 the localities at winch their extreme phases are developed. 



''The Tamias quadrivittatus groupf presents an ecpially, 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- 

 ruginous along the middle of the back, in S. richardsoni it is dull rusty or dark chestnut-browD, and in 

 S.fremonti pale brownish-gray. The prevailing color of the tail in S. hudsonius is nsnally yellowish- 

 rusty, varying to dark ferruginous, with broad annotations of black ; in S. richardsoni, it is black, varied 

 more or less with rusty; in S.frrmonli, black, varied with gray.'' 



"\ Iiimiim quadriviltatus, T.pallasi, T. townsendi, and T. dorsalis of Americau authors." 



