Family GEOMYID^. 



The group to be here treated corresponds to the Sciurospalacoide* of 

 Brandt, the subfamily Geomyina of Baird, the family Geomyuia of Gill, 

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

 milian. Of the former, five species hove been estahhshcd ; of tiie hitter, 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, aiid myisclf, 

 these animals have been united with Dijiodomyn, Perognathus, and Heteromys, 

 in a single family, usually called Saccomyida, 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, nntl 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 iii general aspect, is 

 undoubtedly n safe clue 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 arc shared by the two groups (being the basis of 

 the superfamily Sacromyoidea 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 ba/e been the animal long afterward first technically 



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