272 



from its neighbors, this same condition occurring in a number of shells taken at the 

 same time and place. 



With the foregoing facts in mind, we naturally infer that the combination of 

 conditions occurring in the upper bay tends to produce certain changes in the physi- 

 ological cycle of ridge deposition, the net result of which is a smaller number of ridges 

 per unit area of the shell than we find to be the case under the conditions prevailing 

 in the middle and the lower bay. This corresponds with the results of table 43 and 

 44, which indicate a smaller number of ridges per unit of time for shells from the upper 

 bay. That is, in the Carquinez Strait region. Teredo deposits ridges farther apart and 

 at wider intervals of time than at stations down the bay. 



A further note of interest on the effect on Teredo of lowered salinity and fluctuat- 

 ing temperature is the difference to be observed between shells at different levels of 

 the same pile in the upper bay. This is well brought out in fig. 108, 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 surface 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 ; see also 

 fig. 100). 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 

 (fig. 108, /, 2 and 3). These facts are entirely consistent with our previous conclusions 

 regarding the effect of lowered salinity and fluctuating conditions, 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. 



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

 owing to a paucity of material from near the 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. 



Variation in the Auricle 



Inasmuch as the variations which occur in the posterior lobe of the shell are 

 rather obvious to the eye but extremely difficult adequately to describe, considerable 

 care has been given to the preparation of figures 109 and 112 (i), in order that they may 

 illustrate as accurately as possible the range of the differences we have observed. 



The position of the auricle, we have noted above, varies between posterior and 

 postero-dorsal. It was at first thought that this might have some definite significance 

 with relation to the effect of en\in>nmental factors; but further study has indicated 

 that the position of this lobe of the shell is in considerable degree determined by the 

 age of the specimen. In younger shells the auricle is normally postero-dorsal; but as 

 It grows by further accretions backward and \entrall\', it is resorted at the dorsal edge, 



