275 



so that its position with reference to the median part of the shell becomes continually 

 more definitely posterior. This is strikingly illustrated in figure 108, 4. 



With regard to the degree to which the auricle is reflected outward at its posterior 

 edge, a condition which becomes more exaggerated in the older shells, it is sufficient 

 to remark that this is an adaptation to prevent its cutting into the viscera with the 

 rocking movement of the shell which accompanies boring; naturally this condition 

 becomes more pronounced in those shells having the longer auricles. 



Shells from the upper bay tended in general to have more prominent auricles 

 than shells from the middle and the lower bay. We undertook, accordingly, to get a 

 definite numerical expression of the degree of this difterence. The greatest length of 

 the auricle (measured outside, from the depression which marks its junction with the 

 middle lobe to its posterior edge) was divided by the length of the middle lol)e, as 

 measured on a line taken from the angle between anterior and middle, perpendicular 

 to the forward margin of the middle (fig. 110). The resulting quotient was taken 

 as the index of the relation of the auricle to the middle lobe. These indices were then 

 tabulated and used as the basis of a graphic representation of the range of the differ- 

 ences in question. 



Fig. 110. Diagram of Teredo shell, showing method of obtaining index of auricle. 



The length of the middle lobe was taken as a constant somewhat arbitrarily, not 

 because it does not vary, but for convenience, and because it was deemed a safer 

 constant than, for example, the entire length of the shell, which would include the 

 possibility of an independent 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 (fig. 109) ; 

 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. 



The following graph (fig. Ill) is based upon 100 shells from each of the three 

 localities selected. 



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

 ence 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 intermediate between those from 

 the upper and the middle bay. While all three of our curves manifest certain irregu- 

 larities, owing doubtless to the limited nimiber of shells considered, at se^•en points 



