222 BULLETIN NO. VII. 
CouEs, E., Monographs of N. A. Rodentia, Geomyide. 
CouEs, E., do Saccomyide (Introduction—passim). 
Cougs, E., Exploration of Colorado river, Part iii, Chapter xii—Ab- 
tract of results of a study of the genera Geomys and Thomomys. 
CovuEs, E., The cranial and dental characters of Geomyid#. From the 
Bull. U.S. Geol. and Geogr. Surv., 1875. 
LICHTENSTEIN, K. W. H., Ueber aeussere Backentashen on Nagethieren. 
Koengl. Akad. Wiss. Berlin, 1825. 
The chief characteristics of the family may also be gathered 
from what is said of the one species which comes especially 
under consideration here. 
GENUS GEOMYS, Rar. 
This genus contains four species which occupy a belt through 
the center of the United States from British America to Cen- 
tral America. It does not extend beyond the Rocky mountains 
on the west, nor reach the Atlantic coast except southwardly,. 
As distinguished from Thomomys, Geomys possesses the fol- 
lowing characters: The upper incisors have a deep median 
groove which may be accompanied by a second fainter one 
near the inner margin. The inferior incisor is very long, caus- 
ing a slight protuberance on the outside of the ascending 
ramus between the angle and the condyle, but this is not so 
highly specialized as in Thomomys. The crowns of the molars 
are not acute exteriorly. Zygomatic arch widest in front, not 
forming a sweeping gradual arch as in Synaptomys. The basi- 
occipital is not greatly narrowed. Interparietal triangular. 
The palatal bone is on two planes with a double excavation 
between. Fore claws greatly enlarged,claws of hind feet spade- 
like. The external ears are inconspicuous. 
The differences upon which the genera are separated are 
minute and almost intangible, but on account of the few species 
of Thomomys may be found permanent. ©The latter genus con- 
tains but two species and one of these is a reduced form found 
only in the unfavorable mountain regions of the west. Indeed, 
itmay be said that the limited size and many of the peculiarities 
of the genus are perhaps correllated with the range in dry or 
otherwise ill-suited regions, and are what might have been pre- 
dicated in the case of a Geomys transferred to the same localities. 
Of the genus Geomys three distinct types occur. The first 
represented by G. bursarius or the common pouched gopher 
with a southern variety in the south Atlantic and Gulf states 
(this form, G. tuza, is frequently ranked as a species) ; the 
second type is that of G. mexicanus, which is the Mexican re- 
