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University of California Publications in Zoology [Vol. 22 



length of the shell, which would include the possibility of an indepen- 

 dent variable in the anterior lobe. 



We would mention further that measurements based on the length 

 of the auricle give an inadequate representation of the actual range 

 of variation encountered ; variation occurs, also, as we have previously 

 stated, in breadth and in shape (pi. 16) ; but it is difficult, for obvious 

 reasons, to reduce these factors to a numerical expression. We have 

 for practical purposes considered simply the prominence of the 

 auricle, and of this the measured length gives us a fair indication. 



Fig. E. Diagram of Teredo shell, showing method of obtaining index of 

 auricle. 



The following graphs (Fig. F) are based upon 100 shells from 

 each of the three localities selected. 



In analyzing these curves, we observe first a constant and rather 

 marked difference between shells from Goat Island and those from 

 Crockett, a difference which is expressed alike in the extremes, the 

 means, and the modes. The shells from the upper bay have a 

 definitely and consistently larger auricle than those from the middle 

 bay. The shells from Dumbarton, as we should have anticipated from 

 our analysis of environmental conditions at that place, stand inter- 

 mediate between those from the upper and the middle bay. While 

 all three of our curves manifest certain irregularities, owing doubtless 

 to the limited number of shells considered, at seven points out of a 

 possible eleven the curve representing shells from the lower bay falls 

 between the other two. 



The most salient feature of these graphs is the closeness with which 

 they agree with the graphs representing the number of ridges per 

 millimeter of the shells from the same three localities (fig. D). While 



