68 



The Eastern is the most extended, reaching from the isothermal line of 

 770 F. north and from the Atlantic Ocean to the elevated plains west 

 of the Mississippi Eiver. Many of its forms extend up the bottoms of 

 the rivers which flow to the eastward through " The Plains." The Cen- 

 tral region extends from the limit of the Eastern as far west as the 

 Sierra :Nevada, and south on the mountains of Nevada, and along the 

 mountains of New Mexico. The Sonoran includes parts of Nevada, New 

 Mexico, Arizona, and Sonora in Mexico. It does not cross the Sierra 

 Nevada, nor the Mojave desert, nor extend into the peninsula of Lower 

 California. It sends a belt northward on the east side of the Sierra 

 Nevada as far as, including Owen's Valley in Eastern California, latitude 

 370, and enters other valleys in Nevada in the same way. It occupies 

 the lower valley of the Rio Grande, and extends into Texas as far as 

 the desert east of the Rio Pecos. It extends southward in Western 

 Mexico as far as Mazatlan. The Lower Californian region occupies the 

 peninsula of that name as far north as near San Diego. 



The peculiarities of these regions are well marked. The two regions 

 included in Eastern North America difter from all the others in the 

 abundance of their turtles and the small number of their lizards. 

 Prolific of life, this area is not subdivided by any marked natural bar- 

 riers. Hence, though its species present great varieties in extent of 

 range, it is not divided into districts which are very sharply defined. 

 The warmer regions are much richer in birds, reptiles, and insects than 

 the cooler ; and as we advance northward many species disappear, while 

 a few others are added. The natural division of the eastern part of the 

 continent is then in a measure dependent on the isothermal lines which 

 traverse it. In accordance with this view, the following districts have 

 been proposed, viz: The Carolinian; the Alleghanian ; the Canadian ; 

 and the Hudsonian. 



The Austroriparlan region includes the Floridan, Louisianian, and 

 Texan districts. It possesses many peculiar genera of reptiles not 

 found elsewhere, while the region north of it possesses none, its genera 

 being distributed over some or all of the remaining regions. The num- 

 ber of peculiar species in all departments of animal life is large. 

 It presents the greatest development of the eastern reptile life. Six- 

 teen genera of Reptiles and eight of Batrachia do not range to the 

 northward, while ninety-nine species are restricted in the same manner. 

 The peculiar genera which occur over most of its area are — 



