1922] Miller: Variations in Teredo navalis in San Francisco Bay 299 



two months comparing markedly, both as to mean and to mode, with 

 those tabulated for Goat Island. For the progressive decrease in rate 

 of growth found at the end of the third month we find no explanation 

 in external conditions. Salinity records are available for every day 

 of this period, and show that the salt content of the water, while 

 fluctuating considerably, never fell below 12 parts per 1000, and was 

 usually above 20 parts per 1000 ; being, in fact, at its highest during 

 the period when the decrease in rate of growth occurred. Our experi- 

 mental work has indicated that Teredo will flourish in salinities con- 

 siderably lower than these. The only explanation, therefore, which 

 recommends itself is that suggested above in connection with a similar 



TABLE 2 

 Kate op Growth op Ridges on Shells of Teredo at Crockett 



Number of ritlges : 



12.1 14.2 



Rate of Growth per Month 



4th. 5th and 



6th months 



(mean) 



2 2 



