330 University of California Puhlicafions in Zoology [Vol.13 



Francisco Bay might be a factor in determining tlie distribution of 

 the mollusks if there were regions of relatively impoverished waters. 

 That such conditions exist is suggested in the recent studies upon the 

 diatoms made by Mr. E. 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 impov- 

 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 Mollusca 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, llrosalpinx 

 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. 



