890 MONOGRAPHS OF NORTH AMERICAN RODESTIA. 



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-postcrior ; 

 the last molar much larger than the others, subtriangular, the anterior and 

 inner borders straight, the postcro-outcr 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 

 soli, especially in winter; coloration without spots or streaks. 



Cynomys is the most highly specialized generic group among the Ameri- 

 can Sciuridce. 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 two 

 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 Spermophilus, 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. 



