Y4 BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



Padre Island Tract. — This comprises the low, sandy islands lying 

 off the coasts of Texas and Tamaulipas, on which peculiar (usually 

 pale colored) mammals exist. 



TamauUpan Subtrofical Tract. — This corresponds to the Rio 

 Grande embayment, a limited area about the mouth of the Rio 

 Grande, in which there is an intrusion of Mexican forms. 



Middle Texan Tract. — This includes the region crossed by the Rio 

 Grande between the Rio Grande embayment and the Eastern Desert. 



Eastern Desert Tract. — This comprises a portion of the Rio Grande 

 Valley and a group or chain of ancient lake basins and desert plains 

 extending, as bisected by the Boundary Line, from the Quitman 

 Mountains on the east to the meridian line of the Boundary 100 miles 

 west of El Paso. 



Elevated Central Tract. — This lies between the last point (Monu- 

 ment No. 53) and the Sonoyta River Valley (Monument No. 163), 

 west of the Cobota and Nariz mountains. This area is more complex 

 than the other tracts, as it contains the high western rim of the con- 

 tinental divide, with altitudinal zones ranging upward from the 

 Lower Sonoran (in the Yaqui Basin) to the lower edge of the Boreal 

 (on the highest mountains) ; but nearly all of it lies within the 

 Austral Zone. 



Yaqui Basin Subtropical' Tract. — Increased intensity of coloration 

 characterizes the mammals from the ^'alleys containing the terminal 

 streams of the great Yaqui River of Mexico, some of which rise on 

 the United States side of the Boundary. 



Western Desert Tract. — This begins at the Sonoyta Valley, at 

 Monument No. 163, and extends to the east base of the Coast Range 

 Mountains (Monument No. 227). It is divided by the great Colorado 

 River. 



Calif ornian Subtropical Tract. — This is characterized by the intru- 

 sion of a few peculiar forms from Mexico, in the Colorado Valley and 

 along the east base of the Coast Range Mountains. 



Pacific Coast Tract. — This occupies the strip of land between the 

 Colorado Desert and the Pacific Ocean (Monuments Nos. 227 to 258) 

 on the Boundary, but extending far to the north and south. 



Santa Barbara Island T'rac^.- -Characterized bj^ very distinct 

 mammal forms, which have a heavy coloration, the reverse of those 

 found upon the islands of the Gulf of Mexico. 



DESCRIPTIONS OF PRINCIPAL COLLECTING STATIONS.a 



These descriptions are followed by a condensed tabular list of 102 

 principal collecting stations (p. 143), where the most essential data 



a Fijr location of the collecting stations see Plate I. 



