220 EXPLORATION OF THE CANONS OF THE COLORADO. 
perfectly well-discriminated species, the early links between which have 
entirely disappeared, leaving the forms as stable as possible. Thomomys is 
still in a transition-stage at present. Setting aside the peculiar T. clusius, 
described .as new beyond, all the known forms of that genus arc still only 
incompletely separated, and the links binding them are plainly before our 
eyes. The genus appears to be making into a number of species, but the 
process is still far from completion. In talking over this singular difference 
in the natural history of these two closely-allied genera with Professor Baird, 
he threw out a suggestion, which, perhaps, may account for the facts. While 
we have no means of knowing which is the older of the two genera, so as to 
compare the rates of progress they respectively made in developing their 
species, yet we may fairly infer, upon geographical considerations, that Geo- 
mys has been longer about it than Thomomys. Geottiys, though found to a 
certain longitude westward, is essentially a form of Eastern North America, 
extended thence to Central America, and it is presumably upon older ground 
than the late deposits in the West, where Thomomys occurs. In the cases 
of many mammals and birds of this continent, it will be recollected there is 
a perfectly stable eastern species of a genus which in the West is represented 
by a number of "varieties.'' Junco, Melospiza, Passerella, Picus, are good 
illustrations. But be the explanation what it may, there is no doubt about 
the fact that Geomys has made itself into five or six firm species before 
Thomomys has succeeded in turning out more than one or two. 
With these few observations by way of introducing the species, I shall 
proceed at once to give an account of them, reserving many details which 
contributed to the result of my investigation, as well as all general considera 
tions respecting the family, for publication in a different connection. 
GENUS GEOMYS. (emend, ex Raf.) 
Mua, sp. SHAW, et al., I. c. infra. 
Crlcetus, sp. DESM., et a?., 7. c. infra. 
Geomys, RAF., Am. Month. Mag. ii, 1817, 45. 
Diplostoma, RAF., op. et loc. cit. (char, plerumq : inept.), ncc RICH. 
Saccophorus, KUIIL, Beitr. 1820, 65. 
Pscudostoma, SAY, Loug'a Exp. R. Mts. i, 1823, 406. 
Ascomys, LIGHT., Abhand. Berl. Acad. 1822-'3, 1825, 20. 
