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Show CHAPTER XIII . .ABS'rR.ACT OF HESULTS OF A STUDY OF THE GENERA GEOMYS AND THOMOMYS.* Bv D1e. El.l,t0'1'1' CoUJr.s, U. S. A. These two genern nrc closely allied; the principal difference being in the character of the sulcation of the superior incisors. In Geomys, the upper front teeth have a deep groove along the front face, at or near the mirldle, with or without a fine groove along the inner margin. In Thomomys, the latter groove exists, but there is no other. The fore feet of Geomys are more decidedly fossorial than those of Thomomys, owing to greater development of the claws. In Geomys, the external cars are a mere rim surrounding the auditory orifice; in Tlwmomys, there is a decided, though small, auricle. The two genera constitute a perfectly natural group, of the grade of a family, which may be callerl Geomyidre. equivalent to the subfamily Geomyirue of Baird, or the Sciuro-spalacoides of Brandt. 'rheir closest affinities are with the Saccomyidre (Dipodomys, Perognathus, &c.), unrler which they have been placerl as subfamilies by Waterhouse ttn1l Bairrl. These authors are certainly right in differing from those who, like Brandt and others, widely rlissociate the two groups; for, as Baird has insisterl, they nrc very closely allied in all essential re~pccts, notwithstanding their remarkable dissimilarity in contour aud ot.hcr su pcrficial points. Still, I rlo not go to the length of the authors named in associating Geomys ami Thomomys with Dipodomys, Perognathus, &c., in ouc; preferring to follow Gill in considering them as a distinct, though the * Dascd ou the material contained in tho N at.ional Musoum, Smitbsouiuu Iustit utiou, Washingtoll , D.(). ~S ClOL |