1918 | Packard: Molluscan Fauna from San Francisco Bay 241 
In this section an attempt has been made to segregate a few of the 
more important factors from the diversified environments in which 
the mollusk lives. Depth as far as the local fauna is concerned does 
not appear to have any great significance in determining the distri- 
bution. The deeper waters of the Golden Gate yield faunas nearly 
identical with those obtained from the shallower. The apparent re- 
striction, in a few instances, to certain bathymetric zones appears to 
be due to other factors. Low salinity certainly prevents the marine 
mollusks from ranging much above the lower end of Carquinez Strait. 
It appears to be the most conspicuous factor in accounting for the 
meagerness of the San Pablo Bay fauna in contrast to those from the 
other divisions of the bay. It is not improbable that low salt concen- 
tration prevents certain species like Spisula catilliformis and Mytilus 
californicus from entering the Golden Gate farther than Fort Point. 
The importance of temperature is more uncertain. The greater per- 
centage of predominately southward ranging species in the outside 
fauna may be due to the more uniform temperature for the year, or 
possibly it is to be correlated with the relatively warmer temperature 
during the winter months, which may later be found to correspond to 
the reproductive periods of the majority of the molluscan species. The 
influence of the bottom upon the distribution of the mollusks is the 
most conspicuous of the factors considered. As has been noted, cer- 
tain forms are capable of living under a variety of conditions of bot- 
tom. Several of them were shown to be able to endure marked ex- 
tremes of temperature and salinity as well. A composite bottom, com- 
prising two or more types of materials, offers a greater variety of local 
conditions, and it supports, therefore, a greater number of molluscan 
species than the other types. It is seen that mud supports the greater 
number of individuals per station, but this may be due in part to the 
fact that the dredge sinks deeper in the softer sediments, and thus 
obtains a more complete representation of the life from such a bottom. 
QUANTITATIVE ANALYSIS OF THE FAUNA 
Forty-three dredge hauls were made within San Francisco Bay 
by means of the orange-peel bucket dredge. Such a type of dredge 
had not previously been used for biological purposes. 
This apparatus has an advantage over the various types of trawl 
dredges in that it permits the taking of large masses of mud from a 
single spot, besides rendering possible the capture of deeply burrowing 
