226 EXPLORATION OF THE CANONS OF THE COLORADO. 
considerable distance. I have not been able to discover any more ; and as 
the same number and position have been found to hold in G. tuza, mexi- 
canus, and hispidus, such is probably the normal case in this genus ; though 
in species of Thomomys I have distinctly recognized six pairs. 
The character of the incisor dentition remains for special notice, as a 
diagnostic feature of the species among all its congeners. Two grooves upon 
each upper incisor always persist distinctly. One of these is a sharp, fine line 
of impression, running along the inner margin of the tooth, about the distance 
of its own width ^from the edge. The other is a much larger, more profound, 
and wider sulcus, which fairly bisects the remaining surface, leaving an equal 
plane area on either side, exclusive of the small portion cut off by the fine 
marginal groove. This main groove varies a good deal in depth and width in 
different specimens, and, moreover, is itself sometimes sulcate ; that is to 
say, this excavation sometimes presents, on the outer side, a fine ridge, which 
marks off a secondary groove within the first. The same thing occurs in the 
single-grooved species castanops, mexicanus, and hispidus. But this supple 
mentary carination of the main groove is not always perceptible, and is gen 
erally liable to be overlooked, it is so fine. When most strongly marked, it 
is just like the inner marginal groove itself; and each tooth seems to consist 
of two similar halves. 
Coloration. Throughout this family, the coloration is general and diffused; 
there are no strong special areas of parti-coloration. Most of the species (if 
not all) of both Geomys and Thomomys occur under two states of pelage, which 
we may call the chestnut and the plumbago. The difference is much like 
that between the gray or cinereous states of young Hesperomys, Neotoma, &c., 
and the brighter-colored adults of the same. But, in the present case, it does 
not appear to be a matter of age, since full-sized, if not mature, specimens 
are plumbago-colored. If the dark style of pelage be not wholly fortuitous 
i. e., pure melanism it is, at any rate, as yet unexplained. To these gener 
alities of coloration, it is to be added, that there is a tendency to indefinite 
albinism of the tail, feet, and parts about the mouth. Whatever the phase 
of coloration may be, the character of the pelage is constant. As in all the 
other species, excepting G. hispidus, the fur is remarkably soft, fine, and lus 
trous, reminding one of that of the mole ; yet not of the short, close, velvety- 
