822 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
approaches Cynomys. While S. empetra deviates toward Otospermophilus, 8. 
richardsoni strongly approaches Cynomys, as do S. spilosoma and S. obsoletus 
to Ictidomys, especially in the general contour of the skull. In Ictidomys, 
there is an approach, through S. mezxicanus, toward Tamias, while S. frank- 
lint, 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. 
Scrurvs. 
OTOSPERMOPHILUS, 
| 
| CyYNoMyYs. | 
IcTIDOMYS. 
| Ancroars. | 
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 an account of these, seo the concluding portions of this memoir. 
