388 U. Ss. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT 
THOMOMYS, Maxim. 
Thomemys, Prince Maxim. Nova Acta Acad. C. L. XIX, 1. 1839, 383. 
Branpr, Beit. Kennt. Saugt. Russlands, 1855. 
Oryctomys, Expoux and Gervais, (in part,) Mag. de Zool. VI, 1836, 23.—Is. Voy. de la Favorite, v, 1839. 
Anterior face of upper incisors plane or slightly convex, and with a fine groove along the inner margin, sometimes 
obsolete ; no groove in the centre. Skull not very massive; zygomata slender. Anterior upper molars with two somewhat 
divaricated lobes, the anterior considerably smaller.and sub-circular, The two middle molars narrowly and transversely 
ovate, the acute pointed end outside. Posterior lower molar nearly circular. Fore feet considerably shorter than the 
hinder ones, comparatively small and weak, with the claws not much developed, though considerably larger than the hinder 
ones. Second finger and claw nearly equal to the fourth. Cheek pouches moderately large. 
The description of the osteology of Geomys will answer very well for that of Thomomys. The 
principal differences are seen in the nearly smooth anterior faces of the upper incisors, which 
have merely a very fine groove along the inner margin and the ovate crowns of the molars, 
which in Geomys are truly elliptical. 
For the present I leave the species of Thomomys without any attempt to subdivide them, or to 
exhibit their characteristics in a brief synopsis. All the species of the family vary exceedingly 
as to color and proportions, especially when these are based on the dried skin. This is exceed- 
ingly extensible, and may thus convey the idea of a comparatively short tail, when, in reality, 
in nearly or quite all the species, the tail is not much less than half the length of the body. 
The size of the claws and teeth vary; the color of the immature, though full grown animal, 
differs from the adult, and, as already stated, the exact proportions can only be ascertained from 
fresh specimens, or those preserved in alcohol. The descriptions in the following pages were all 
made two years ago, and although, with the additional specimens received, a careful revision of 
the whole subject might have furnished more satisfactory results, yet the time allotted for the 
completion of the present report will not admit of the necessary delay. Iam, however, well 
satisfied that I have not materially overstated the number of species described, as whatever may 
be the variations of single specimens from the type, there is an appreciable difference in the 
series, even though it be difficult of intelligible expression. 
The species to be described are as follows : 
1. Thomomys bulbivorus.—Coast of California, from Tejon Pass to some distance north of San 
Francisco. 
2. Thomomys laticeps.—Coast of northern California, (Humboldt Bay.) 
3. Thomoniys douglassit.—Lower valley of the Columbia River, and Puget’s Sound. 
4. ? Thomomys borealis.—Upper valleys of the Columbia, towards Rocky Mountains; proba- 
bly at higher elevations than 7. douglassii. 
5. T'homomys rufescens.—Upper Missouri and Saskatchewan. 
6. Thomomys talpoides.—Shores of Hudson’s Bay. 
7. Thomomys wmbrinus.—Western Texas and New Mexico, along eastern slope of Rocky 
Mountains, and along the mountains into Sonora. 
8. Thomomys fulvus.—Valley of the Colorado and tributaries, from the San Francisco moun- 
tains to Fort Yuma, and across to San Diego. 
