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U. S. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
of Waterhouse in combining Geomys and Thomomys into a family with Perognathus and Dipo- 
domys. Perognathus he considers rather as a Muroid, and coming next to Oricetus, while 
Dipodomys, or rather Macrocolus, is placed as the type of a sub-family Macrocolini under the 
Dipodoides. I think, however, a revision of the subject, with more ample materials before him, 
will satisfy this eminent zoologist of the soundness of Waterhouse’s view. The common 
characters of external cheek pouches, five distinct fingers each, with a separate claw on the 
fore feet, the stiff rigid hair, and the many common osteological peculiarities, are quite enough 
to separate them from all others, while in the essentials of structure they agree quite as well 
among themselves as do the agile and graceful long-tailed squirrels and the short-tailed marmots, 
or the various extreme structural types of the Muridae. 
Sub-Family GEOMYINAE. 
Skull massive; incisors very large and thick. The ante-orhital foramen small, appearing far forward on the side of the 
muzzle. The nasals extended but little beyond the incisors. Mastoid bone restricted to the occiput. Occipital bbne 
broad, forming the posterior portion of the skull or the occiput. Body thick-set and clumsy. Limbs about equal, and all 
very short; fore claws enormously developed, five in number. 
The above characters are among the most striking as distinguishing this family from the 
Saccomyinae, although many others will be found detailed in the descriptions of the genera 
themselves. The species of Geomyinae, in fact, are as different in appearance from the Sacco¬ 
myinae as is well possible ; the one being clumsy, short limbed, squat, almost entirely subterra¬ 
nean, and so strictly nocturnal, that many persons who have daily seen hundreds of their 
mounds have never in their lives observed the living animal; while the typical Saccomyinae are 
graceful, agile, and with a development of limb and tail every way equal that of the Jerboas; 
the individuals themselves well and familiarly known in their native localities. 
Some further characteristics of the Geomyinae are found in the very great contraction of the 
skull between the orbits, which there is narrower than the snout. The palate is horizontal only 
between the molars; anteriorly it rises very rapidly to near the incisors, leaving a deep 
concavity between the molars and incisors. Its plane, likewise, is much below that of the base 
of the cranium. Th e meatus auditorius externus is a tube projecting forward and embraced in 
a notch of the squamous portion of the temporal. 
The genera Geomys and Thomomys , composing the Geomyinae , are very closely related, and in 
fact differ comparatively little among themselves ; the points of distinction from others are, how¬ 
ever, for the most part, very appreciable. All are confined to North America and Mexico, and 
in certain localities west of the Mississippi are very abundant. 
The difficulties in determining the species of Geomys and Thomomys have always been very 
great, owing to various causes. In the first place, there is the same trouble in procuring speci¬ 
mens of this as of all other subterranean animals, as they rarely see the light, and sometimes 
are never observed even by persons who have lived for years in districts quite densely inhabited 
by them. They vary, too, with age and season to a considerable extent, so that in the small 
number of specimens seen by naturalists, at least of the far western species, each one has almost 
served as the type of a distinct species. There is a very strong resemblance among all the mem¬ 
bers of the genus, and owing to their peculiar conformation, the points of structure affording 
good distinctive features are easily distorted or obscured. The best characters are, perhaps, to 
