248 THE MOLLUSK FAUNA OF NORTHWEST AMERICA. 



types are represented by a much greater number of species and profusion of 

 individuals in that area than elsewhere; and that as we increase our distance from 

 the area mentioned the types are represented by fewer and less varied forms* 

 then the northwest coast faunal region is undoubtedly the center of distribution 

 for such types as Buccinum, Chrysodomus, IAomesus, Beringius, and Nucella 

 all of which are represented there, as in no other region. The richness of the 

 fauna is particularly striking when it is compared with that of the northeast coast 

 of the continent. 



There are very few if any species which can be traced with probability 

 as immigrants from the Southern Hemisphere or the Atlantic. The Indo- 

 Pacific admixture in the West American tropical fauna is almost negligible. So 

 far as yet known the proportion is less than one tenth of one per cent. 



So far as paleontological evidence is yet available, the intimacy between the 

 faunas of the Panamic and Caribbean seas which existed in the Oligocene rapidly 

 diminished after their separation by the elevation of the isthmus between them; 

 nearly all the characteristic west coast types becoming extinct on the Atlantic 

 side before the present epoch. Many genera originally common to both oceans 

 are now represented by analogous species on the Pacific side, but less abundantly 

 or not at all on the Atlantic shores. 



Taking a broad general view of the west coast mollusk fauna of North America 

 as a whole, we come to the conclusion that it is of all marine mollusk faunas the 

 least intimately related to any other existing fauna. This may be supposed 

 to be due to the broad expanse of ocean separating it, except at its northern and 

 southern extremes from any other land, and to the necessarily slow rate of 

 distribution of molluscan life over the floor of the deep sea. 



