218 EXPLORATION OF TOE CANONS OF THE COLORADO. 
most nearly allied, family. The Saccoinyidce are extremely lithe, agile, graceful 
animals ; jerboa-like, with long saltatorial hind limbs, elongated and often 
tufted tail, large ears, and full eyes, and are not specially nocturnal or subter 
ranean in. habits. The Geom.yidce, on the other hand, are hamster-like, or 
rather an exaggeration of that kind of structure ; they are among the heaviest 
for their inches of any animals of this country, of squat, bunchy shape, with 
short, thick limbs, a short tail, very small or rudimentary ears, small eyes, no 
appreciable neck, and thick, blunt head ; and they are as completely subter 
ranean as the mole itself. They are rarely and only momentarily seen above 
ground ; they excavate endless galleries in the earth in their search for food, 
frequently coining to the surface to throw out the earth in heaps, but plug 
ging up these orifices as soon as they have served their purpose. 
Both families agree in possessing enormous cheek-pouches, overlying 
the whole side of the head, in some species even reaching over the neck and 
shoulders. The nature and construction of these sacs was long misunder 
stood. They were supposed for many years to be external pendulous bags 
opening into the mouth, and thus to differ only in degree of development from 
the ordinary "cheek-pouches" of many other rodents an enlargement of the 
mucous membrane of the mouth and skin of the cheeks. But, as now well 
known, they have no connection with the mouth ; at least, no more than the 
abdominal pocket of an opossum has with the genitalia. Their chief purpose 
is not even related to the food of the species ; they are sacs that the animals 
use chiefly in carrying out dirt from their burrows to deposit it on the surface 
of the ground. They are fully described beyond. 
Several circumstances have conspired to obscure the history of the 
Geomyidce, and to involve the determination of the species in doubt. In the 
first place, the animals are l-argely withdrawn from ordinary observation, and 
the acquisition of specimens is difficult. Their geographical distribution is 
limited to a portion of America. Very few specimens, comparatively, have 
ever reached Europe, and very few foreign naturalists have written about 
them from anything like sufficient means of observation. In fact, they are 
among the rarest sets of specimens in any museums ; and I think it probable 
that there are before me, as I write, more prepared specimens than have 
before been examined by all naturalists put together. This shows the 
