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 et at. 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. 



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 faeies. 



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. 



Kelation to the Available Food 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 



