Introduction 



of the Sierras. It is greatly broken into by the colder and less 

 dry Boreal and Transition zones that extend southward in the 

 high altitudes and moister valley regions of the mountains. 



The whole subregion is made up of arid elevated plains and 

 plateaus, of desert regions and mountain ranges. It is notable 

 for its lack of Batrachia, the only genera of the Salientia at all 

 well represented being Bufo and Scaphiopus. There is no species 

 common to the whole Sonoran subregion. Unlike the Eastern 

 Austral zones, it presents great natural barriers to the spreading 

 of animal races, and so is cut into divisions, each having its more 

 or less peculiar fauna. 



That part of the subregion east of the Rocky Mountains is 

 grass-covered, but relatively dry and treeless. It not only has 

 an arid atmosphere, but it is so elevated that the temperatures 

 are low. It has but few species of Salientia; two almost peculiar 

 to it are Bufo cognatus and Scaphiopus hammondii bombifrons. 



In Texas this dry region is separated from the moist Austro- 

 riparian subregion of the eastern one-third of the state by the 

 boundary of the first plateau (one of the series of three steppes 

 which present a terraced rise westward from near sea-level to 

 4,000 feet above). This line passes near Fort Worth, Austin, 

 and San Antonio, Texas, to the valley of the Rio Grande. In 

 topographical, climatic, and faunal conditions, western Texas is 

 as different from eastern Texas as Colorado is from Missouri. 

 Austroriparian Texas is a plain with low hills of red sand and 

 with shallow valleys and ravines. It is largely forest-covered, 

 has a great fertile prairie in the central region, and many swamps 

 in its southern portion. It is well watered also by the moist 

 winds from the Gulf. In sharp contrast with this, the Sonoran 

 portion — especially the region of the first plateau — is of 

 peculiar interest. It is dry because of its elevation and because 

 all prevailing winds are depleted of their moisture in crossing 

 eastern Mexico or eastern Texas. It is picturesque with deep 

 forest canyons. The lower levels are covered with buffalo grass 

 and with various species of cacti and yuccas. Species peculiar 

 to this district are Liihodytes lairans and Syrrophus^ marnockii. 

 But from the surrounding sections of country the following long 

 list of species is gained: Hyla cinerea, Smilisca baiidinii, 

 Chorophilus n. iriseriaius, Rana areolata, Rana pipiens, Engysioma 



* Lithodytes and Syirophus are both Mexican genera. 



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