1918] Packard: Quantitative Analysis of Molluscan Fauna 327 
Curves similar to those just considered suggest the importance of 
the temperature factor. The mean annual temperature curve as pub- 
lished by Sumner ef al. appears to have little significance when com- 
pared with the specimen curve. Table 9 indicates, however, that the 
larger number of living individuals per haul were obtained in regions 
of relatively low annual temperatures. 
TABLE 10 
Average 
Mean number 
annual Number living 
temperature Groups of hauls individuals 
11.98-12.35 1 6 134.5 
12.36-12.73 2 13 41.0 
12.74-13.11 3 14 42.4 
13.11-13.49 4 6 3.3 
13.50-13.88 5 4 0.0 
It appears from figure E that those portions of the bay where the 
seasonal range of temperature is high are regions in which the areal 
density is relatively low. It is not certain, however, that this indicates 
a causal relationship. 
The correspondence between the October and July temperature 
curves and that of the specimen curve indicates that more mollusks 
occur within a given area where the waters are cooler during those 
months. In this connection it is of interest to note that the bay fauna 
includes a majority of predominantly northward ranging species. 
However, it is not evident that the warm summer temperatures of 
the other divisions of the bay act as a barrier to these northern forms, 
for the open ocean during this period is cooler than that of the bay 
and yet it has a fauna showing a southern facies. 
If the temperature factor is important in determining the local 
distribution of the mollusks, the greater areal density of the middle 
division of the bay may be due to the low seasonal range or to the 
low summer temperature. 
This rather indefinite relationship between molluscan distribution 
and temperature may indicate that this factor is effective only during 
the reproductive periods of a particular mollusk. If these periods 
do not all fall within a single season, as seems rather improbable, it is 
not surprising that the influence of temperature is obscure. 
RELATION TO THE AVAILABLE Foop SuPPLY 
The plankton probably serves as the most important food supply 
of the pelecypods, which in turn become the main supply for the 
predaceous gastropods. The distribution of the plankton within San 
