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648 



MONOGUAPnS or KORTH AMERICAN RODENTIA, 



mens from the Black Hills. This form becomes even still pnlcr and more 

 fulvous at the eastern base of the main chain of the Eocky Mountains, be- 

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

 gradation into the so-called Sciurus rkhardsoni of the Rocky Mountains 

 north of 45°, and the so-called Sciurus fremonli of the Rocky Mountains 

 soulh 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 {8. hudsonius, S. rich'ardsoni, 

 and S.fremonti), whose habitats here meet, that it is impossible to soy 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 ricnc-dsoni and fremonti 

 forms on the other. To the southward of this district we soon pass into tlie 

 region of the typical //•cwjowii, and to the westward and northward into the 

 liabitat 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 tlie 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. douglassi meets the range of (he 

 irne 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 groupf presents an equally, or even more, 

 striking range of variation in color, and also varies to an unusual degree in 



"'While the prevailiog color above in 8.\»dtonl»t if) light yelloviab-brown, varylog to bright fer- 

 rnginous along the middle of the back, in <S. rickardtinti it is dull rnsty or dark cheatnut-browD, and ia 

 S.fremonti pale brownish-gray. The prevailing color of the tail in 8. \»iioniiu ii nsnally yellowisli- 

 rnsty, varying to dark ferruginous, with broad annnlatioos of black ; in 8. riokardsoai, it is black, varied 

 more or less with rusty ; in S.fremooli, block, varied with gray." '' 



"\TanUu gNadricUlalM, r.jiallati, T. Uncnunii, and T. iorialii of American aotbon." 



