Family GEOMYID^. 



The group to be here treated corresponds to the Sciuroxpalacoides of 

 Brandt, the subfamily GeomyitMb of Baird, the family Geomyidce of Gill, 

 including only the two genera Geomys of Rafinesque and Thomomys of Maxi- 

 milian. Of the former, five species have been established ; of the latter, two. 

 These animals are confined to the cold-temperate and warmer portions of 

 continental America, from portions of British to Central America; their 

 centre of distribution being the United States from the Mississippi Valley to 

 the Pacific Ocean. 



By all late authors, so far as I know, excepting Brandt, Gill, and myself, 

 these animals have been united with Dipodomys, Perognathus, and Heieromys, 

 in a single family, usually called Saccomyidce, a leading character of which is 

 possession, unique among Mammals, of external fur-lined pouches on the sides 

 of the head, not directly connected with the mouth, and thus of entirely 

 different construction and relations from the ordinary cheek-pouches of vari- 

 ous Rodents. The classification afforded by these singular organs, which 

 are present in two groups of animals widely different in general aspect, is 

 undoubtedly a safe c]ue to natural affinities, since it is borne out by the gen- 

 eral structural relationships of the animals ; these being so close, that, after all, 

 no great violence would be done by embracing the two groups under a single 

 family. Yet their respective peculiarities seem sufficient to justify, if not to 

 require, the recognition of two separate families. 



The characters in question need not be here discussed nor even formally 

 presented, as those which are shared by the two groups (being the basis of 

 the superfamily Saccomyoidea of Gill), as well as those in which the two 

 groups differ, have been presented on foregoing pages of this work, where 

 the general bearings of the case are further discussed. 



The history of the family begins at a very early (pre-Linnsean) period, 

 though nearly everything relating to it is of much later date. The "Tucan" 

 of Hernandez appears to have been the animal long afterward first technically 



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