1918] Packard: Molluscan Fauna from San Francisco Bay 245 
The three bathymetrie zones represented in these waters are not 
divisible into subzones upon the basis of the molluscan life. 
The large proportion of the predominantly southward ranging 
species are restricted to the waters of the open ocean, where the winter 
temperatures are higher than they are in the bay. 
The influence of salinity may be noted within Carquinez Strait, 
where the low salt concentration acts as an effective barrier to the 
marine species. The salinity factor is considered to be the major one 
in accounting for the meagerness of the fauna from the upper in 
contrast to that from the other divisions of the bay. 
The character of the bottom appears to have the greatest signifi- 
cance in determining the local distribution of the mollusks. The 
stations at which both precise bottom data and faunas were obtained 
are grouped into seven classes, each representing a particular type of 
bottom. The lists of prevalent species for these bottom groups shows 
that the distribution of several of the species is unrelated to any par- 
ticular type of bottom. There are other species, however, that are 
characteristically found upon one of these types of bottoms. 
A bottom of pure sand or pure mud is not as favorable to a varied 
mollusean life as is one comprising two or more types of materials. 
A mud bottom appears to support a larger number of living individ- 
uals than does any of the other types. Sand is shown to be the least 
favorable to an abundant mollusean life. 
CATALOGUE OF SPECIES 
This catalogue includes the forms taken by the Survey and those 
that have previously been reported from San Francisco Bay and the 
adjacent waters outside the Golden Gate. The synonyms given in- 
clude the name under which the species was described and the more 
common ones found in the literature dealing with the Mollusea of the 
San Francisco region. The dimensions given represent the maximum 
and the minimum lengths of all of the specimens obtained by the 
Survey. The italicized number in the parenthesis following the num- 
ber of the dredging station indicates the number of living specimens 
obtained at that station, while the arabic number indicates the number 
of valves in the ease of the peleeypods, or the number of specimens 
of other shell-bearing mollusks that were dead at the time of the 
dredging. In a few cases the specimens were not counted, only an 
estimated number being given. Occasionally a fragment that could 
be determined specifically is recorded by the letter ‘‘f’’. 
