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 between 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.