1922] 



Schmidt, Amphibians and Reptiles of Lower California 



627 



piceus, and Salvadora hexalepis) which reach the Cape area without 

 having been differentiated from the northern stock. 



The Lower Californian amphibians and reptiles are accordingly- 

 divisible into groups or "faunal elements" which seem to represent suc- 

 cessive invasions from the continental fauna at the north. These may 

 be arranged in a very tentative order as shown in the accompanying 

 table. 



I. The "Mexican" forms 



Middle or Pre-Miocene 



Early Tertiary immigration 



Extensive Destruction of Fauna by Igneus Activity 



Last Extensive Submergence 



The most important consideration bearing on the history of the 

 Lower Californian fauna is the discontinuity of the ranges of numerous 

 genera and species at the Colorado Desert to the north, which appears 

 to be inserted like a wedge separating closely related forms in Arizona 

 and Lower California. It may be supposed that an increasing aridity 

 in this area during Pleistocene and post-glacial time has led to the evolu- 

 tion of forms specifically adapted to desert conditions in this area, their 

 ancestors, where they continue, being forced to follow their migrating 



