Famiy GEOM YID AS. 
The group to be here treated corresponds to the Sciurospalacoides of 
Brandt, the subfamily Geomyine of Baird, the family Geomyide 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 Heteromys, 
in a single family, usually called Saccomyide, 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 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 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-Linnzan) 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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