COUES ON GEOMYS AND THOMOMYS OSTEOLOGY. 275 
of distinct character in the two genera, furnishing the most ready means 
of diagnosis, not only of the genera, but of the species of Geomys, as already 
fully given in' -the body of this^paper. 
The molars are perennial rootless prisms, as in Arvicolince and many 
other hard gnawers, but are small and of a very simple structure at least 
in comparison with the compfteate character which obtains in many rodents. 
The whole mojar series is scarcely one-seventh of the length of the skull. 
They are implanted very obliquely to suit the peculiar conformation of the 
parts. The axis of the anterior upper molar slopes backward at an angle of 
about 45, and the rest succeed with regularly-diminishing obliquity. The 
relation is reversed in the lower jaw, where the Tjack molar slopes forward, 
the rest becoming successively more nearly perpendicular. There is the 
same number of teeth in both jaws, and they are quite similar in construction. 
The anterior molar in each jaw is a double prism ; the others are single and 
simple, elliptical in cross-section, the first being a pair of ellipses laid together 
like a short broad figure-of-eight, and the last approaching a cylindrical figure. 
The relation of the molars to each other is somewhat singular. Their roots 
are all widely diverging, but their crowns come into close contact. This is 
effected by the curve in -their axis. Thus the front upper molar is curved 
with the convexity posterior ; the rest are curved successively more and more, 
with the convexity anterior. Similar characters mark the under molars, 
though less strongly; and there is seen in these teeth, especially in the anterior 
ones, a lateral as well as fore-and-aft curve. This shape appears to be forced 
upon the teeth by the peculiar conformation of the alveoli. The molars are 
quite similar in the two genera, and scarcely afford diagnostic characters, 
especially since there is some change in the details of the molar crowns with 
age and wear of the teeth. On the whole, however, it may be observed that 
in Geomys the molars the immediate ones, at any rate are more perfectly 
elliptical than they are in Thomomys, where a pinching-together of the exte 
rior portion of the ellipses tends to result in a pyriform contour. 
The principal cranial and dental characters of the two genera which com 
pose the Geomyidte may be shortly contrasted, as follows : 
