1918] Packard: Molluscan Fauna from San Francisco Bay 235 
adaptable forms of the entire fauna. Their general distribution 
throughout the bay shows that they can adjust themselves to diverse 
environments. It is not surprising, therefore, to note that they have 
a wide geographic range, two being cireumpolar in distribution. 
The true brackish water fauna of San Francisco Bay has not been 
investigated in this Survey. It is not unlikely that the species Ceri- 
theridea californica would be found to be restricted to brackish water, 
since it occurs in the salt marshes bordering the bay and was not taken 
by the Survey in the bay proper. 
The mollusean fauna from San Pablo Bay is meager in comparison 
to that from the other divisions of the bay. The prevalent species 
here are among those listed as prevalent also within the lower and 
middle segments. However, a number of those that are prevalent in 
the latter regions are not abundantly represented in the upper division. 
Cardium corbis, for instance, was taken at only four hauls within the 
latter division. A glance at the distribution charts of the more com- 
mon species (pls. 42 to 59) shows that in the majority of cases the 
species are represented as extending up into the lower end of San 
Pablo Bay. This distribution may be best explained by the salinity 
factor. The other factors are not equally applicable, for neither the 
character of the bottom nor the temperature changes so markedly in 
the vicinity of Point San Pablo as do the curves for salinity. (See 
Sumner et al., 1914.) 
The distribution of Mytilus californicus, Spisula catilliformis, and 
Tellina bodegensis within the outer portion of the Golden Gate is not 
as easily explained. These forms are known to be abundant in the 
adlittoral zone along the open ocean. There the temperatures are, 
for the most part quite similar to those given above for the waters off 
San Francisco. Such conditions probably prevail to a lesser extent 
in the outer portion of the Golden Gate. 
INFLUENCE OF THE CHARACTER OF THE Bottom 
The character of the bottom is a factor of considerable importance 
in determining the distribution of mollusks. It is evident that solid 
objects are necessary for the attachment of sessile pelecypods such as 
Ostrea, Hinnites, or Monia. A support of some sort is essential to the 
limpets, and usually to the chitons. Mud may interfere with the 
respiratory currents of some species, while it represents the optimum 
habitat of certain burrowing forms. Thus a close correspondence be- 
