SUMMARY OP THE MARINE SIIELLBEARING MOLLUSKS 

 OF THE NORTHWKST COAST OF AMERICA, FROM 

 SAN DIEGO, CALIFORNIA, TO THE POLAR SEA, MOST- 

 LY CONTAINED IN THE COLLECTION OF THE UNITED 

 STATES NATIONAL MUSEUM, WITH ILLUSTRATIONS 

 OF HITHERTO UNFIGURKD SPECIES. 



By William Healey Dall., 

 Honorary Curator of Mollusks, United States National Museum. 



INTRODUCTION. 



To the preparation of this summary the author has brought the 

 results of more than 50 years' study of the molluscan fauna of the 

 northwest coast, his personal investigations in the field having begun 

 in 1865, when, under the auspices of the Smithsonian Institution, he 

 visited the shores of the Pacific as a member of the scientific corps of 

 the Western Union Telegraph Expedition. 



Since that time a constant inflow of material from that region has 

 enriched the collection of the United States National Museum until 

 it has become unrivaled in this group of animals. 



Very few of the species described from the area between the 

 southern boundary of the United States and the Polar Sea on the 

 Pacific coast are absent from the series referred to. In a few cases 

 where the writer has not been able to verify personally the names or 

 the distribution from authentic specimens, the name of the authority 

 upon whom the data rest has been added in parentheses. 



The distribution of the Mollusca on the west coast of America may 

 be allotted to three grand divisions : the Arctic or Boreal, the Tem- 

 perate, and the Tropical faunas. 



The first, containing many Circumboreal 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 Conception, California. 



The Tropic division extends from this point southward to Point 

 Aguja on the coast of Peru. 



Each' of these divisions may be subdivided into subordinate and 

 reasonably distinct faunas; the Arctic into Eastern and Western, or 



1 



