ORDER III. 
RODENTIA. 
Incisor teeth two in each jaw, (a second rudimentary pair behind the upper incisors in the Leporidae,) very largo, with sharp 
cutting chisel shaped edges, fitted for gnawing. No canines, but a wide space without teeth between these and the molars. 
The condyle of the lower jaw longitudinal, rounded, having free motion, longitudinally, in the glenoid cavity. 
The Rodenlia are unmistakably characterized by the incisor teeth as given above, although 
in some other features they are easily distinguished from the remaining orders of mammals. 
They exist in all parts of the world, and are especially abundant in America, which contains 
nearly as many species as all the rest of the world put together. South America counts, 
however, more species than the northern half of the new world, the preponderance being 
caused principally by the large number belonging to the genus Hesperomys, of which our little 
deer—or white-footed wood mouse is a familiar example. 
There are no indigenous Rodents common to Europe and North America, unless we except 
the beaver, which is by some authorities supposed to be the same. 1 Spermophilus parryi and 
Fiber zibethicus, or the muskrat, are, however, said to occur in the northeastern part of Asiatic 
Russia, nearest the continent of America, especially on the peninsula of Kamtschatka and 
islands adjacent. The statement is more certain in respect to the former than to the latter. 
Nor is there evidence that any North American species are found in South America, although 
a number extend into Mexico. There is, however, a close relationship between their families 
and genera, as will be seen by the following brief discussion of the distribution of the genera 
and species of the North American types. 
Taking, the families up in the order: Sciuridae, Saccomyidae, Muridae, Hystricidae, and 
Leporidae, we find the 
Sciuridae especially abundant in the United States, which embraces within its limits nearly 
one-third of the known species, at least of the nominal ones. This development is particularly 
seen in Sciurus and Spermophilus, species of Pteromys being more numerous in Asia, and none 
occurring in South America. A few species of Sciurus are found there, but thus far no Sper¬ 
mophilus . Tamias is almost peculiar to North America ; two species only (perhaps but one) 
occurring in Siberia. The genus Castor extends over the whole of North America and into 
Mexico, specimens having been received by the Smithsonian Institution from Tamaulipas, as 
well as from numerous points along the whole extent of the Rio Grande and Gila rivers. The 
same species, or a variety, occurs through the north of the Old World, although it is only on 
the Obi of Siberia that it constitutes an item in the fur -trade. Aplodontia, the last genus of 
this family, is confined, as far as known, to Washington Territory. 
Saccomyidae. —This small but natural group, well entitled to the rank of a distinct family, 
is, as far as known, confined entirely to America. Of the six component genera, ( Macrocolus 
being a synonym of Dipodomys ,) Heteromys is found in Trinidad and Central A merica, Saccomys 
probably in the West Indies, and Dipodomys, Perognathus, Thomomys, and Geomys in Mexico 
and North America. Dipodomys, though abundant in Mexico as far north as the Rio Grande, 
' Some authorities consider the lemmings of America identical with European species. 
