1206 REPORT OF NATIONAL MUSEUM, 1898. 



ously pointed out, to the great difference in rainfall between the part 

 of the continent lying eastward of the one-hundredth meridian and 

 that i^art which lies west of it. This difference is coincident with a 

 profound difference in geologic age between the regions west of that 

 meridian and the eastern district, the former having a short conti- 

 nental history as compared with the latter. 



I, however, agree with Merriam in the abolition of the "Central" as 

 a subregion of Medicolumbia. 



The relation of the several zoological divisions to these subregions 

 are as follows : The eastern subregion is the original center of distri- 

 bution of all the fishes peculiar to the Medicoliimbian region, except 

 only the Plagopterine Cyprinida'. It is the center of distribution of all 

 the Batrachia, with the following exceptions: The degenerate types of 

 Trachystomata and Amphiamidte probably originated in the A.ustro- 

 riparian subregion, and the species of Bufo in the Sonoran. The east- 

 ern snl)region is also the source of the aquatic Testudinata. On the 

 other hand the Sauria of the eastern and Austrorij)arian subregions 

 are ail overflow from tlie abundant lizard life of the Sonoran legion, 

 excepting the family of tlie skinks, and the genus Anolis, the latter 

 being of ]S"eotro])ical origin. The snakes also are mainly Sonoran 

 types, including especially the true rattlesnakes. The copperheads 

 and ground rattlesnakes are on the contrary indigenous to the eastern 

 subregion. The Pacific subregion has close affinities with the Sonoran, 

 but of a largely different kind as to its lizards, while the Batrachia 

 have the character of the eastern types as far as they go. 



The distribution of types indicates six i^rincipal subdivisions, which 

 I call the Floridan, Austroriparian, Eastern, Sonoran, Western, and 

 Toltecan subregions. The Floridan subregion includes the greater 

 part of the peninsula of Florida, being bounded approximately on the 

 west by the Suwanee River. The Austroriparian subregion extends 

 northward from the Gulf of Mexico to the isothermal of 77° F. It 

 begins near Norfolk, Virginia, and occupies a belt along the coast, 

 extending inland in North Carolina. It ])asses south of the Georgia 

 mountains, and to the northwestward up the Mississippi Valley to the 

 southeastern part of Illinois. West of the Mississippi the bouudsirv 

 crosses Missouri, extends south along the southern boundary of high 

 lauds of Texas, and reaches the Gulf at the moutli of the Eio Grande. 

 The Eastern suhngion is the most extended, reaching from the iso- 

 thermal line of 77° F. north and from the Atlantic Ocean to the elevate d 

 plains west of the Mississip])i River. Many of its forms extend up ti e 

 bottoms of the rivers which flow to the eastward through the plains. 

 The Sonoran subregion extends from the limit of the Eastern as fur 

 west as the Sierra Nevada, and south, including Nevada, New Mexico, 

 Arizona, Sonora, and the Plateau of Mexico, including the State of Chi- 

 huahua, and, i)erhaps, Durango. It does not cross the Sierra Nevada, 

 but includes the entire peninsula of Lower California. It extends north- 



