ANCIENT PORTALS OF TEE EARTH 399 



a southern arm of the Oriental Tethys carried the same assemblage of 

 forms down to South Africa. 



The early Tertiary Eocene epoch witnessed the reopening of the 

 Central American portal, and the immigration of the Mediterranean 

 Venericardia planicosta fauna into the Pacific waters of the Californian 

 province. This is the last true interregional zone in the history of the 

 earth, and it is the last time when widespread warmth of the waters 

 made such distribution possible. Since that time the Central American 

 and the Asia Minor portals have been closed, and the continually 

 increasing demarcation of climatic zones have prevented migration be- 

 tween the greater sea basins through northern waters. The later 

 Tertiary epoch witnessed the inauguration of modern conditions, and 

 the geographic regions were restricted as they are now. 



