INTRODUCTION 



In the preparation of this checklist the author has iitiHzed 

 the results of almost exacth' fifty years study of the molluscan 

 fauna of the Pacific Coast, his personal investigations ha\ ing 

 begun in May, 1865, shortly after his arrival at San h^rancisco. 



Experience has shown that the possession of a satisfactory 

 checklist is one of the most ef^cient means of promoting the 

 study of the fauna of any given area. 



Moreover, in the 'case of the mollusca it is a fact that nearly 

 all the modern textbooks arc out of print and can only be had 

 at second hand, and also that in the time which has elapsed 

 since their publication the reformation of the nomenclattire has 

 made unprecedented progress, shown in a number of more or 

 less scattered memoirs or essays, difificult to bring together. 



The classification of the bivalves adopted here is the result 

 of a study of all their known characters and therefore dififers 

 from the conclusions of one or two European writers based 

 upon single or physiologically unimportant features. It is doubt- 

 less doomed to submit to future improvement as our knowledge 

 of the anatomy of these animals is increased, but may claim 'to 

 embody the results of the most modern investigations to the 

 present time. 



In a list of this kind it is impossible to give synonyms ex- 

 cept in the most urgent cases. The addition of the year in 

 which the original description appeared, however, will assist 

 the student having access to the literatiu'e to supply this for 

 himself. 



• The distribution given is that authenticated from actual 

 inspection of specimens l)y the writer, except in a few cases 

 where the authority is given in parentheses. Doubtless many 

 readers of the checklist will be able to enlarge the range here 

 printed. Communication of such data for future use is greatly 

 desired by the author, and may be addressed to him, care of 

 the Smithsonian Institution, Washington, D. C. 



The distribution of the ]\Iollusca on the West Coast of 



