1896,] Distribution of Batrachia and Reptilia. 895 
to the great difference in rainfall between the part of the con- 
tinent lying eastward of the 100th meridian and that part 
which lies west of it. This difference is coincident with a pro- 
found difference in geologic age between the regions west of 
that meridian and the eastern district, the former having a 
short continental 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 sub- 
regions are as follows: The eastern subregion is the original 
centre of distribution of all the fishes peculiar to the Medico- 
lumbian region, except only the Plagopterine Cyprinide. It 
is the centre of distribution of all the Batrachia, with the fol- 
lowing exceptions: The degenerate types of Trachystomata 
and Amphiumoidea probably originated in the Austroriparian 
subregion, and the species of Bufo in the Sonoran. The eastern 
subregion is also the source of the aquatic Testudinata. On 
the other hand the Sauria of the eastern and Austroriparian 
subregions are an overflow from the abundant lizard life of the 
Sonoran region, excepting the family of the skincs, and the 
genus Anolis, the latter being of Neotropical 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 principal 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 commences near 
Norfolk, Va., and occupies a belt along the coast, extending in- 
land in North Carolina. It passes south of the Georgia Mount- 
ains, and to the northwestward up the Mississippi Valley to the 
southeastern part of Illinois. West of the Mississippi the bound- 
. 
