88 



appears weak and sliglit in comparison with the enormous incisors. The 

 under incisors, as already said in effect, run the whole length of the 

 jaw, and push up a kuoh of bone behind, They are of the ordinary 

 scalpriform construction, quite flat-iaced, with converging* sides and 

 beveled to an edge behind. The superior incisors describe nearly a 

 semicircle through the intermaxillaries and far into the maxillaries, to 

 below the root of the zj^goma. They are of distinct character in the 

 two genera, furnishing the most ready means of diagnosis. In Tho- 

 momys the faces are plane, with a very fine groove running immediately 

 along the inner margin ; this groove frequently being indistinct or 

 altogether wanting. In Geomys, on the other hand, this fine groove 

 may or may not co-exist with another much stronger one which deei:)ly 

 channels the face of the tooth. This is as in Bipodomys and Ferogna- 

 thus of Saccomyidce, Ochetodon, Beithrodon, and Synaptomys of Muri- 

 dee, &c. The position of this main groove even distinguishes species of 

 Oeomyidce. In G. hursarhis it is approximately central, and is accom- 

 panied by the fine marginal line of impression. In G. pineti it is rather 

 more exterior, and the marginal groove may become obsolete. In G. 

 castanops and G. mexicanus, there is no trace of an inner fine groove, 

 and the main one exactly bisects the face of the tooth. In G. liispidus, 

 finally, the main groove, which is not accompanied by a marginal one, 

 lies in the inner moiety of the tooth. 



The molars are perennial rootless prisms, as in Arvicolincc and many 

 other hard gnawers, but are small and of a very simple structure — at 

 least in comparison with the complicate character which obtains in many 

 rodents. The whole molar 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 regu- 

 larly diminishing obliquity. The relation is reversed in the lower jaw, 

 where the back 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 convexit^' posterior ; the rest are curved succes- 

 sively 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 pecu- 

 liar 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 ThomomySj where a pinching together of the 

 exterior portion of the ellipses tends to result in a pyriform contour. 



The principal cranial and dental characters of the two genera which 

 compose the Geomyidce may be shortly contrasted, as follows: 



■ GEOMYS. 



Superior incisors deeply channeled 

 along the middle, with or without 

 a fine marginal groove. 



THOMOMYS. 



Supeiior incisors without median 

 sulcus, but with a tine marginal 

 groove, (sometimes obsolete.) 



