71 



The cbaraeteristics of the fish-fauna of Eastern Nearctica are much 

 more marked; two entire orders, represented by the gar {Ginglymodi) 

 and dog-fish [Ualecomorplii), are confined to it, and a series of genera of 

 Fercklae, embracing many species, known as Etheosfominae, have the 

 same range. The Siluridae all belong here, as well as a great majority 

 of the genera of Cyprtnulae and Catostomidae. In all of these divisions, 

 tlie region is very rich in species, owing to the abundance of everflow- 

 ing rivers and streams which drain it. The Polyodontldae (spoon-bill or 

 paddle-fish) are not found in any of the other regions.* 



The Central region is characterized by the general absence of forests, 

 as compared with the Eastern. It presents two distinct divisions, each 

 peculiar in its vegetation : the division of the plains, which extends from 

 the eastern border to the Rocky Mountains; and the Rocky Mountain 

 region itself, which extends to the Sierra Nevada. The former is covered 

 with grass, and is almost totally treeless; the latter is covered with " sage- 

 brush.''' {Artemisia), a short stout bush, which forms extensive areasof tree- 

 less brush. The grass-covered plains are the range of tlie bison, though it 

 formerly sought also the tracts of grass occasionally found among the 

 Artemisia. The region, as a whole, is distinguished from the Eastern 

 by the possession of several genera of ruminating Artiodactyles, i. e., An- 

 tilocapra, Haplocerus, and v is, as well as certain species of the same group, 

 i. e., Cariacus macrotis (black-tailed deer) and C. leucunis. Other genera 

 of mammals which distinguish it from the Eastern are Taxidea, Cynomys, 

 SpermopJiilus, Dipodomys, Perognatlms, and Lagomys. A few species 

 of Spermophilxis extend into the northwestern portion of the Eastern ; 

 while the extensive genus Geomys (the subterranean gophers) range 

 over the Central subregion, and into the Western and Gulf States 

 the Austroriparian as far as the Savannah River. A great many species 

 of birds are i^eculiar to the Central region, and the following genera : 



Oroscoptes. 



Hydrobata. 



Myiadestes. 



Keocorys. 



Salpincies. 



Picicorvus. 



Cliondestes. 



{Jalamospiza. 



Emhernagra. 



Centrocercus. 



Pedioecetes. 



Excepting the coui-se of the Mississippi, and perhaps the Kio Grande. 



