6 INTRODUCTION 



America may be allotted to three grand divisions ; the Arctic 

 or Boreal, the Temperate, and the Tropical faunas. 



The first, containing many Circumborcal species, extends 

 from the Arctic or Polar Sea to the southern limit of drift ice 

 in winter in Bering Sea. The Temperate division extends from 

 this line southward on the west coast of America to Point Con- 

 ception, California. The Tropic division reaches from this point 

 south to Point Aguja on the coast of Peru. Each of these 

 divisions may be subdivided into subordinate and reasonablv 

 distinct faunas ; the Arctic into eastern and western or Atlantic 

 and Pacific ; the Temperate into Aleutian, ( )regonian, and Cali- 

 fornian ; the Tropic into the Gulf of Californian, the Panamic, 

 and the Ecuadorian. 



Taking the distribution of a single family these distinc- 

 tions might have their boundaries modified, but taking the 

 average of the whole molluscan fauna, thev appear to be w^ell 

 established. 



The limits of these faunas are not in all cases sharply de- 

 fined. Some of the less sensitive species will be found to range 

 far beyond their divisional boundaries ; and their range is 

 largely governed by the temperature of the water as distributed 

 by the ocean currents. 



In southeastern Alaska, where the inshore channels are 

 kept cold by the drainage from glacier streams, many Arctic 

 forms persist, while on the outer fringe of the archipelago, in 

 warmer water, quite a number of the more southern species 

 extend their range northward. 



The student of this list will notice many more species, which 

 from California extend their range to Panama or even Peru 

 than are recorded in any previous publication. 



The general discussion of the limits of the several faunas 

 and subfaunas must await the revision of the gastropod mollusks 

 now in progress. 



I have included the species of deep water dredged near the 

 coast (although the distribution of the deep water fauna is 

 altogether dififerent from that of the shore) because, being sub- 

 ject to the sea temperatures many of those found at a depth of 

 hundreds of fathoms at the south, in the cold water of the 



