318 University of California Publications in Zoology [Vou. 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 mollusean life. 
REATION TO DrprH 
The effect of depth upon the distribution of the Mollusea 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 RELATIVE 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, im which most of the deeper hauls 
were made. 
RELATION TO TyPE or Borrom 
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 
