1922] Miller: Variations in Teredo navalis in San Francisco Bay 309 



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 ade- 

 quately to describe, considerable care has been given to the prepara- 

 tion of plates 16 and 17 (fig. 1) 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 posterodorsal. It was at first thought that this might 

 have some definite significance with relation to the effect of environ- 

 mental 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 ventrally, 

 it is resorbed at the dorsal edge, so that its position with reference 

 to the median part of the shell becomes continually more definitely 

 posterior. This is strikingly illustrated in plate 15, figure 4. 



With regard to the degree to which the auricle is reflected out- 

 ward at its posterior edge, a condition which becomes more exagger- 

 ated 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 promi- 

 nent auricles than shells from the middle and the lower bay. We 

 undertook, accordingly, to get a definite numerical expression of the 

 degree of this difference. The greatest length of the auricle (meas- 

 ured outside, from the depression which marks its junction with the 

 middle lobe to its posterior edge) was divided by the length of the 

 middle lobe, as measured on a line taken from the angle between 

 anterior and middle, perpendicular to the forward margin of the 

 middle (fig. E). 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 differences in question. 



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 



