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. 



