890 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
thrown outward basally, the bony plate forming their outer border being 
oblique to the vertical plane of the skull; upper premolars two, the first large, 
of the same breadth antero-posteriorly as the other grinding teeth (except- 
ing the last molar); dentition heavy, the grinding teeth (except the first pre- 
molar and last molar) with the transverse breadth twice the antero-posterior ; 
the last molar much larger than the others, subtriangular, the anterior and 
inner borders straight, the postero-outer rounded ; molar series strongly con- 
vergent posteriorly ; cheek-pouches moderate ; tail very short, flattened ; ears 
rudimentary ; pollex large, with a well-developed nail; pelage short, full, and 
soft, especially in winter; coloration without spots or streaks. | 
Cynomys is the most highly specialized generic group among the Ameri- 
can Sciuride. It is especially characterized by its massive dentition, which is 
heavier than that of any other Sciuromorph with which I am acquainted, and 
by the great posterior convergence of the upper molar series. In other fea- 
tures, it differs little from Colobotis, possessing only the characters of that 
group, particularly as represented in its most specialized phase (for instance, 
as in our Spermophilus [Colobotis| richardsoni and in S. [ Colobotis| brevicauda 
of Siberia), in an exaggerated degree. The grinding teeth, however, are set 
more obliquely in the jaw (those of the upper jaw being directed strongly 
outward, and those of the lower jaw in the opposite direction), while the 
teeth themselves are relatively shorter and broader, with three instead of twa 
transverse grooves in the outer half of the triturating surface (in the maxil- 
lary series), The palate is slightly narrowed posteriorly in Spermophilus, but 
in Cynomys its breadth between the last molars is less than half its width 
between the first premolars. The general outlines of the skull, both in pro- 
file and as seen from above, differ little from those of Spermophitus, the differ- 
ence in this respect being not greater than between different species of either 
the subgenera Colobotis or Ictidomys. The occipital, interparietal, and tem- 
poral crests are, however, much more strongly developed than in any of the 
Spermophili, and all the processes and ridges of the lower jaw are much 
more prominent. The general form of the lower jaw is quite similar to that 
of Spermophilus, especially as met with in the heavier forms of the subgenus - 
Colobotis ; the coronoid process, however, is shorter, stouter, and has a much ~ 
greater antero-posterior breadth, and the angular process is more produced. 
The mental foramen is placed lower than in most species of Spermophilus 
and Arctomys, but not lower than in some of them (S. brevicauda for example), 
its position being just midway between the upper and lower edges of the jaw. 
