4 
BULLETIN 112, UNITED STATES NATIONAL MUSEUM. 
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 ;md 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 distribuCon. Probably no tw^o persons would 
agree exactly with all such decisions, but there seems to be no way 
to avoid this difficulty. 
Excluding the Cephalopoda and Nudibranchiata which I have left 
for treatment by those more expert in those special groups, and add- 
ing a few species wdiich 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. s 
1,424 
Scaphopoda 
19 
Polj^placophora 
_ 134 
Pteropoda 
20 
Aplacophora 
16 
Heteropoda _ _ 
13 
O 1 
Geographically distributed as follows: 
Species introduced with “ seed ” oysters 6 
Pelagic species 28 
Exclusively abyssal species 151 
Arctic species 148 
Species o :' the Aleutian subfauna 291 
Species of the Oregonian fauna 371 
Species of the Californian fauna 996 
Species of the Pauainic fauna 131 
Total species 2,122 
