222 BULLETIN NO. VII. 



COUES, E., Monographs of N. A. Rodentia, Geomyidae. 



COUES, E., do Saccomyidas (Introduction 



COUES, E., Exploration of Colorado river, Part iii, Chapter xii Ab- 



tract of results of a study of the genera Geomys a ad Thomomys. 

 COUES, E., The cranial and dental characters of Geomyidae. From the 



Bull. U. S. Geol. and Geogr. Surv., 1875. 

 LICHTENSTEIN, K. W. H., Ueber aeussere Backentashen on l>agethieren. 



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, RAF. 



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, 

 it may 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 thre'e 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- 



