28 Merrlam — Geographic Distribution of Life. 



genus (r<'oiiii/>i inhabit both these divisions on the Great Plains 

 and in the Mississii)pi Valley, and range east to tlie Atlantic in 

 the Austroriparian Zone. 



Both divisions of the Ijower Sonoran are inhabited by the 

 transcontinental genera Reithrodontomys, Sigmodon, Corgnorhinus, 

 Ngdinxnnus, Otnpterus, Neotoma, and SpilogaU, though in the west 

 the two last mentioned range through the Upi^er Sonoran also. 



The humid Lower Sonoran or Austroriparian is a division of 

 much importance. It begins on the Atlantic seaboard at the 

 mouth of Chesapeake Bay and stretches thence southwesterly, 

 embracing the alluvial lands of the South Atlantic and Gulf 

 States below what geologists know as the ' fiill line,' rising in the 

 Mississippi bottom as far as the junction of the Ohio with the 

 ]Mississip})i, and following the former in a narrow strin 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 sjireads out over Indian and Oklohoma 

 Territories and Texas, where it loses its moisture and merges 

 insensibly into the arid Sonoran. Orgzomgs and Ngcticejus are 

 distinctive Austroriparian genera. Six other genera {Neotoma, 

 R.eithrodontomy», Geovigs, Spilogale, NgctinoinuSj and Corgnorhinus), 

 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 (Sigtnodon), another characteristic Austroriparian 

 genus, lias 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 Ari- 

 zona south of the plateau rim (sending a tongue up the Rio 

 Grande to a point above Albuquerque), the west side of which 

 it follows northerly to the extreme northwestern corner of Ari- 

 zona and the southwestern corner of Utah (where it is restricted 

 to the valley of the lower Santa C'lara, or St. George Valley), and 

 thence westerly across Nevada, reaching northerly to Pahranagat, 

 Oasis, and Owens Valleys, and thence curving southwesterly, 

 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 

 mountains. It sends an arm southward over most of the penin- 

 sula of Lower California, and anotlier northward covering the 

 San .Joaquin and Sacramento Vallevs. The distinctive mammals 



