330 University of California Publications in Zoology [Vou.18 
Francisco Bay might be a factor in determining the distribution of 
the mollusks if there were regions of relatively impoverished waters. 
That such conditions exist is suggested im the recent studies upon the 
diatoms made by Mr. H. P. Rankin. He shows that the number of 
species and individuals of these plants decreases as one passes from 
the middle to the upper division of the bay, and that the marine forms 
are not there replaced by fresh water species. The main channel 
through that arm of the bay is found to represent a region of impoy- 
erished water in comparison to that of the quieter and presumably 
more saline water near shore. 
This distribution of the diatoms is paralleled in general by that 
of mollusks, as is shown by the relative number of species and indi- 
viduals per haul for San Pablo Bay in comparison with the other 
regions of the bay (see p. 18). However, this apparent correlation 
is probably not due to the lack of food supply, for Professor Kofoid’s 
studies show that the plankton of the bay is relatively rich; it is more 
probably due to the salinity, which is exceedingly variable within that 
region. It is thus evident that from the data at hand no definite 
conclusion regarding the relationship of the distribution of the plank- 
ton to that of the mollusks can be reached. 
RELATION TO THE Biotic ENVIRONMENT 
The relation between the distribution of other forms of life and 
that of the Mollusea can only vaguely be suggested. From the stand- 
point of the food it seems that the distribution of the plankton when 
present in quantities above the requirements of the organism has little 
influence upon the occurrence of the mollusks. Until the Algae of 
this region are better known it is impossible to say that certain of the 
gastropods are not distributed according to the occurrence of certain 
of these plants. The distribution of some of the predaceous gastropods 
corresponds to that of their prey. Unfortunately no quantitative data 
are available regarding the distribution of the oyster drill, Urosalpinz 
cinereus, but qualitative studies show that it occurs most abundantly 
upon the oyster beds. The relation of the enemies of the mollusks 
and the distribution of several gastropods the shells of which are 
inhabited by hermit crabs can only be ascertained by a detailed study 
of the entire fauna and flora of the bay. 
