324 University of California Publications in Zoology [Vol. 13 



It is not improbable that the annual range of salinity is even more 

 potent in determining the abundance of mollusks than is the mean 

 annual salinity. A comparison of the specimen curve with the pub- 

 lished curve indicating the annual range of salinity (Sumner et al., 

 fig. N, p. 69) shows only a general increase in the number of mollusks 

 with decrease in range of salinity, for two of the highest points of the 

 specimen curve fall within the area of high range in salinity. 



The curves showing the distribution of salinities in the bay during 

 April 23 to May 6 corresponds more closely with the specimen curve 

 than does any of the others representing the salinities at other periods 

 of the year. 



The highest average number of mollusks per haul is found at those 

 stations having a mean annual salinity between 28 and 30 per mille. 

 If this represent the optimum salinity for the bay fauna, that portion 

 of the bay having a salinity most nearly that of these figures should 

 yield the largest number of mollusks per unit area. No portion of 

 the upper bay satisfies such a condition, but the middle division does 

 fulfil such a requirement and is also the richest faunally. The lower 

 bay is found to hold an intermediate position both faunally and from 

 the standpoint of salinity. However, since such a salinity is the rule 

 in the middle portion of the bay and to a lesser extent in the lower 

 division this apparent relationship may have but little significance. 

 If the optimum mean annual salinity is high, as seems reasonable, it 

 might be expected that the regions where the salinity is low at any 

 period of the year will be low in the number of mollusks per unit area. 

 Thus the inverse relationship shown in the curves (figures A and C) 

 might have been foretold. It appears, then, that minimum salinity is 

 one of the factors influencing the distribution of the local mollusks. 

 The closer correspondence between the specimen curve and the mini- 

 mum seasonal salinity curve than between any of the other curves 

 showing the salinity for the other periods of the year tends to confirm 

 such a statement. 



Kelation to Temperature 

 It is not improbable that the moUuscan larvae are more susceptible 

 to temperature control than is the adult mollusk. An investigation 

 of the water temperatures during the periods of reproduction is de- 

 sirable from the standpoint of the oyster culture as well as from that 

 of pure science. Unfortunately data as to the reproductive periods 

 of the local species are not available. Therefore only the more con- 

 spicuous effects of temperature can at present be determined. 



