8386 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
know as the “ fall line,” rising in the Mississippi bottom as far as the 
junction of the Ohio with the Mississippi, and following the former in 
a narrow strip to the point where it receives the Wabash. On the west 
side of the Mississippi it crosses Arkansas, reaches southern Missouri 
and southeastern Kansas, and spreads out over Indian and Oklahoma 
Territories and Texas, where it loses its moisture and merges insensibly 
jnto the arid Sonoran. Oryzomys and Nycticejus are distinctive Aus- 
troriparian genera. Six other genera (Neotoma, Reithrodontomys, 
Geomys, Spilogale, Nyctinomus, and Corynorhinus), which in the region 
east of the Mississippi seem to be restricted to this division, have a 
more extended range in the west. The cotton rat (Sigmodon), another 
characteristic Austro-riparian genus, has a very limited range in the 
arid Sonoran. 
The arid Lower Sonoran extends westerly from the humid Sonoran 
to the Pacific, covering southern New Mexico and Arizona south of the 
plateau rim (sending a tongue up the Rio Grande to a point above Albu- 
querque), the west side of which it follows northerly to the extreme 
northwestern corner of Arizona and the southwestern corner of Utah 
(where it is restricted to the valley of the lower Santa Clara, or St. 
George Valley), and thence westerly across Nevada, reaching northerly 
to Pahranagat, Oasis, and Owens valleys, and thence curving south- 
westerly, following the eastern base of the Sierra Nevada, Tehachapi, 
and Tejon Mountains, and covers the whole of the Mohave and Colo- 
rado deserts and all the rest of southern California except the moun- 
tains. Itsends an arm southward over most of the peninsula of Lower 
California, and another northward covering the San Joaquin and Sac- 
ramento valleys. The distinctive mammals of the arid Lower Sonoran 
are kangaroo rats of the genus Dipodomys, pocket-mice of the sub-genus 
Chetodipus, and spermophiles of the sub-genera Xerospermophilus and 
Ammospermophilus. 
The peninsula of Lower California is a sub-division of the arid Lower 
Sonoran zone. Not a single genus of land mammal or bird is restricted 
to it and but two peculiar species of mammals have been described. 
The peculiar birds are more numerous, but with few exceptions are 
only sub-specifically separable from those of neighboring parts of the 
United States and Mexico. They may be classed in two categories: 
(1) mountain forms derived from the North (of Boreal or Transition 
origin); and (2) lowland forms derived from the contiguous plains (of 
Sonoran, or in one instance, sub-tropical origin). As would be ex- 
pected from the character of the country, the great majority are sub- 
species of well known Sonoran forms, with the addition of a small num- 
ber of peculiar species belonging to Sonoran genera. But a single sub- 
tropical bird is known, namely, Dendroica bryanti castaneiceps, and it 
is restricted to the mangrove lagoons. 
The presence of this sub-tropical bird in the narrow coast lagoons is 
in complete accord with the vegetation of the coast strip, which, as Mr. 
