308 University of California Publications in Zoology [Vol. 22 



This is well brought out in plate 15, figure 1, where we have compared 

 a series of shells taken at Crockett from near the mud line of a pile 

 (depth 20 feet) with a series from near the top of the same pile, 

 taken at about low tide level. Our hydrographic data show that the 

 differences between top and bottom salinities at this point, owing to 

 the greater specific gravity of the more saline water, may at times 

 be as great as 6 parts per 1000 (Kofoid and Miller, 1922, p. 81). 

 We know further that in this location conditions sometimes prove 

 lethal to Teredo near the top of a pile, while permitting it to survive 

 lower down. In other words, the surface conditions here are very 

 close to the critical point which determines the life or death of the 

 animal. 



The shells of Teredo occurring in this region of stress at the upper 

 end of the pile are very notably dwarfed as compared with those 

 from a greater depth. Furthermore, they have fewer ridges, and 

 the interspaces separating these are considerably wider (pi. 15, figs. 1, 

 2 and 3). These facts are entirely consistent with our previous con- 

 clusions regarding the effect of lowered salinity and fluctuating con- 

 ditions, being merely an exaggeration of the differences found between 

 shells from Crockett and Goat Island. 



It is to be remarked also that shells from near the surface at 

 Crockett are very similar to shells from near the bottom at Port 

 Costa and Martinez, points farther up the bay, -where Teredo leads 

 a precarious existence and rarely survives the fresh-water period of 

 winter floods. 



Under these unusual conditions the estimate of the rate of growth 

 of the shell of Teredo set down above (p. 300) would not hold good, 

 growth being much retarded. 



Elsewhere in the bay the effect of depth has not been taken into 

 consideration, owing to a paucity of material from near tide levels. 

 A few specimens that we have from the upper part of a pile in the 

 middle bay are not observably different from those taken at greater 

 depths. Conditions at surface and bottom are much more uniform 

 in the middle and the lower bay, and any effect of depth is probably 

 so slight as to be negligible. 



