CHAPTER XIII. 
ABSTRACT OF RESULTS OF A STUDY OF THE GENERA 
GEOMYS AND THOMOMYS* 
BY DK. ELLIOTT COUES, U. S. A. 
These two genera are 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 middle, 
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 ears are a mere rim surrounding the 
auditory orifice ; in Thomomys, there is a decided, though small, auricle. The 
two genera constitute a perfectly natural group, of the grade of a family, 
which may be called' Geomyida, equivalent to the subfamily Geomyiruz of 
Baird, or the Sciuro-spalacoides of Brandt. Their closest affinities are 'with 
the SaccomyicUe (Dipodomys, Perognathus, &c.), under which they have been 
placed as subfamilies by Waterhouse and Baird. These authors are certainly 
right in differing from those who, like Brandt and others, widely dissociate the 
two groups ; for, as Baird has insisted, they are very closely allied in all essen 
tial respects, notwithstanding their remarkable dissimilarity in contour and 
other superficial points. Still, I do not go to the length of the authors named 
in associating Geomys and Thomomys with Dipodomys, Perogtmthus, &c., in 
one ; preferring to follow Gill in considering them as a distinct, though the 
'Based on tho material contained in the National Museum, Smithsonian Institution, Washing 
ton, D. C. 
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