ORDER III. 



RODENTIA. 



Incisor teeth two in each jaw, (a second rudimentary pair behind the upper incisors in the Leporidae,) very large, with sharp 

 cutting chisel shaped edges, fitted for pnawing. 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 Rodentia 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 tipermophilus 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, Nuridae, 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 America, 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, 



1 Some authorities consider the lemmings of America identical with European species. 



