822 



MONOGRAMS OF NORTH AMERICAN RODENTIA. 



approaches Cynomys. While S. empetra deviates toward Otospermophilus, S. 

 richardsoni strongly approaches Cynomys, as do 8. spilosoma and S. obsoletus 

 to Ictidomys, especially in the general contour of the skull. In Ictidomys, 

 there is an approach, through S. mexicanus, toward Tamias, while S. franh- 

 llni, in its full, broad, and rather long tail and prominent ears, and somewhat 

 in the general shape of the skull and rather small first upper premolar, rather 

 inclines toward Otospermophilus. 



As is evident from the foregoing remarks, the complex inosculations of 

 Sciurus, Tamias, Cynomys, and the several divisions of Spermophilus, render 

 it impossible to arrange them serially in a single line. The following dia- 

 grammatic disposition to some extent expresses their mutual affinities, as well 

 as their relationship to Arctomys. 



Sciurus. 



Otosper moph I LU8. 



Tamias. 



Cynomys. 



Ictidomys. 



Arctomys. 



The absence of any broad gaps among the many species composing the 

 above-named groups leads to the inference of their comparatively recent diverg- 

 ence from some common ancestral type, which may or may not have been 

 something nearer Arctomys than Sciurus, or something quite different from 

 either. The extinct species of the Tertiary formations,* of which there are 

 indications of a large number, none of which, unfortunately, are as yet well 

 known, were mainly of large size, most of them exceeding the existing 

 species of Cynomys, while some equalled and even exceeded the largest species 



* For au accouut of those, boo tho concluding portions of this memoir. 



