318 University of California Puhlications in Zoology [Vol.13 



viduals being nearly seven times as great. These differences as brought 

 out in this table challenge investigation as to their causes. 



There is such an inter-relation between the different factors that 

 determine the distribution of animals of this class that it is difficult 

 to determine the effect of any single one upon the molluscan life. 



Reation to Depth 

 The effect of depth upon the distribution of the Mollusca is probably 

 insignificant within these local waters. This conclusion is based more 

 largely upon a study of the distribution of the entire fauna collected 

 by the Survey than upon the results of this study. However, the 

 following table is presented in which the averages per haul for four 

 different bathymetric zones are given. 



TABLE 7 



The Eelative Abundance of Species and Individuals for Different 

 Bathymetric Zones 



Average 



number 



of living Average number of 



Depth in Number individuals species per haul 



fathoms of hauls per haul Pelecypoda Gastropoda 



0to5 2 5.5 3.5 0.0 



5 to 10 26 42.5 4.57 1.1 



10 to 15 .12 31.8 5.5 1.1 



15 to 20 3 ^ 152.6 5.3 .6 



This table suggests that the number of living individuals per 

 dredge haul is greater with increase of depth. A similar correspond- 

 ence is seen in the last column in table 6, where the greatest average 

 depth for the quantitative hauls occurs in the middle division of the 

 bay, which is there shown to be the richest faunally. This apparent 

 bathymetric distribution may be due to other factors which are pecu- 

 liar to the middle portion of the bay, in which most of the deeper hauls 

 were made. 



Relation to Type op Bottom 

 The character of the bottom is a recognized factor in determining 

 the distribution of mollusks. In order to show the relative abundance 

 of molluscan life on different types of bottoms, the following table 

 has been prepared. Seven types of bottom have arbitrarily been 

 recognized. This classification is based upon the physical analyses of 

 the bottom samples, supplemented by the notes regarding the bottom 



