50 THE LIFE OF THE MOLLUSCA 
conditions. A great majority of the Lower Tertiary 
(Eocene and Oligocene) genera still exist, but none 
of the species. During the succeeding Miocene a 
few species, which are still in existence, made their 
appearance, while of the Pliocene species 80 or go 
per cent. are represented in the recent fauna. 
At the close of the Eocene the wide distribution of 
many types now characteristic of warm, temperate, 
or tropical waters, began to be restricted, and during 
the Miocene the faunal boundaries of the Mollusca 
were mapped out nearly on existing lines. This was 
more true of the non-marine forms; but not till the 
Pliocene did each geographical province come to 
assume its present distinctive features. 
