236 
U. S. P. R R. EXP. AND SURVEYS-ZOOLOGY-GENERAL REPORT. 
does not appear to cross this river into Texas, except in the western part of the State; it extends 
along the Rocky Mountain range—certainly as far as 38° N., and west to the Pacific, where it 
reaches nearly to the Columbia river. Perognathus, with similar limits on the whole is found 
on the Missouri river as far east as Fort Union, Nebraska, though it is not known whether it 
occurs in a direct line between this point and Matamoras, where it has been detected by Dr. 
Berlandiere and Lieut. Couch. 1 Geomys has a more general distribution, being found in 
Georgia and the Gulf States, and from the region bordering the Mississippi, to the Rocky Moun¬ 
tains, and even almost to the Arctic Sea on the north, and to Central America on the south. 
All the pouched rats hitherto collected west of the Rocky Mountains, however, belong to the 
section Thomomys, which is limited eastward by the Mississippi. 
Muridae. —Of this, the most extensive family of the Rodents, the North American represen¬ 
tatives are not very numerous, being confined principally to the Arvicolinae. The Dipodinae are 
illustrated by the genus Jaculus, with one or two species, most abundant in the northern part 
of the continent, and extending to the Pacific. The Ctenodoctylinae are confined to Africa. Of 
the Murina we have no Old World Mus; our few species of mice belonging to the American 
group of Hesperomys , which, in a small number of species, is found all over the continent to a 
very high latitude. It is in South America that the Hesperomys, in several subdivisions, reach 
their highest development, eighty or more species being enumerated. Of Eeithrodon, distin¬ 
guished from Hesperomys by the grooved upper incisor, and likewise found in South America, 
there are only a few North American species, belonging to the southern States and the Rocky 
Mountains. The Neotoma, with thin tails, are found in the southern States, and through Texas 
and northern Mexico—perhaps southern California—to the Pacific; more sparingly in the 
western States. A few specimens of N. floridana have been caught by Mr. John G. Bell near 
the city of New York, far out of their usual range. The hush-tailed Neotomas occur on the 
head waters of the Missouri and west to the Pacific ocean, probably also in California; their 
extreme northern range has not yet been ascertained. An extinct species, N. magist.er, Baird, 
much larger than any of those now existing, was formerly very abundant in Pennsylvania, as 
shown by numerous remains found in the hone caves about Carlisle, Harrisburg, and other 
localities. Sigmodon has much the same southern and southwestern distribution as Neotoma, 
though there is as yet no evidence of its occurrrence west of the Rocky Mountains. 
Of the remaining sections of Muridae, viz: Spalacinae, Batliyerginae, and Arvicolinae, the 
latter alone occurs in North America, where it is very fully represented by the genus Arvicola , 
with numerous species spread all over the continent. Here also belong Myodes, confined mainly 
to the Arctic circle, and Fibei', distributed as widely as Arvicola . The two first mentioned 
genera are found in Europe, but not at all in South America, nor scarcely in Mexico ; the last 
is found only in North America. 
Hystricidae .—Of this family, North America possesses but one genus, Eretliizon, of a single 
sub-family, Hystricinae, and probably but a single species, or at most two ; of upwards of eighty 
species enumerated in the family, but four are found in the Old World, the rest being South 
American, with the single northern exception just stated. Among the South American Hystri¬ 
cidae there is found the largest of living Rodents, the Capybara, though this, in turn, is much 
exceeded in size by the extinct Gastoroides, represented in North America by C. ohioensis. 
Leporidae .—The numerous species of hares belonging to the genus Lepus may be said to 
characterize North America above all other countries, twelve or more being enumerated. They 
1 Since this was written a species has been detected at Fort Riley, Kansas. 
