4 BULLETIN 112, UNITED STATES NATIONAL M.USEIIM. 



from Mexican localities) when they went collecting on California 

 beaches, and sometimes spilling a few to be picked up later by other 

 collectors, is responsible for some early records which with better 

 knowledge seem adventitious. The Monterey beaches furnished sev- 

 eral records of this kind to Gabb and Cooper, which I have rejected 

 as doubtful. Another source of error arose from the small vessels 

 which used to cruise in the Gulf of California and on the Mexican 

 coast, and which arriving at their home ports in California were 

 beached to scrape off accumulated seaweed, barnacles, etc., containing 

 a certain number of southern mollusks and brachiopods, which, how- 

 ever, never seem to become acclimated. San Diego has furnished some 

 records which may be attributed to this cause. However, eliminating 

 such sources of error, the collections in the United States National 

 Museum upon which this list is primarily based, show a much larger 

 proportion of species ranging from Point Conception to the Gulf of 

 California, Mexico, and Panama than was thought possible by Car- 

 penter and other early students of the West Coast faunas. 



In endeavoring to assort according to their respective faunas the 

 species in the following discussion I have met with the difficulty of 

 properly placing a number of species which overlap the limits of one 

 or more faunal districts in their geographic range. In such cases I 

 have tried to assign them to the fauna which seemed to be their 

 natural center of distribution. Probably no two persons would 

 agree exactly with all such decisions, but there seems to be no way 

 to avoid this difficulty. 



Excluding the Cephalopoda and Nudibranehiata which I have left 

 for treatment by those more expert in those special groups, and add- 

 ing a few species which have come to my notice among the bivalves 

 since the publication of my check list of the Pelecypoda, I find the 

 census giving the following results for the region included : 



Pelecypoda— 496 Gastropoda s. * 1,424 



Scaphopoda 19 rolypla.oiihora 134 



Pteropoda _______ 20 Aplacophora 10 



Heteropoda— _ 13 



Total species 2. 122 



Geographically distributed as follows: 



Species introduced with "seed" oysters G 



Pelagic species 28 



Exclusively abyssal specie-- _ 131 



Arctic species 1-8 



Species Oi the Aleutian subfauna. 291 



Species of the Oregonian fauna _. 371 



Species of the Californian faumi ______ 996 



Species of the Panainic fauna 131 



Total spec;*-- .._.___... — 2, 122 



