1922] Miller: Variations in Teredo navalis in San Francisco Bay 301 



VAKIABILITY AND ENVIRONMENT 



There is scarcely a detail of the structure of a Teredo shell which 

 does not exhibit variability. The outlines of the shell and the rela- 

 tions of the various parts to each other are frequently markedly 

 dissimilar in different specimens. This is particularly true with 

 reference to the auricle, which may be reduced or very prominent, 

 rounded or quadrate, or elongate and reflected ; its position varies 

 between posterior and posterodorsal. Other marked variables are the 

 prominence of the ridges on the anterior and median lobes and the 

 width of the spacing between them, details of the serrations on these 

 ridges, width of the anterior median denticulated area, size of the 

 angle formed at the junction of anterior and median lobes, degree of 

 convexity of the shell, and thickness and color of the periostracum. 

 On the interior of the shell, considerable variation occurs in the rela- 

 tive length and breadth of the apophysis, and in the width of the 

 shelf produced by the slight overlapping of the inner edge of the 

 auricle upon the median portion. 



To treat in detail of all the deviations observed in these studies 

 would be a practical impossibility, as well as inconsistent with our 

 limited knowledge of the conditions which occasion them. We have 

 accordingly confined attention to those variations which are at once 

 the most salient and the most easily described or expressed in terms 

 of numerical coefficients. Such are the variations occurring in num- 

 bers of ridges, in length of auricle, in certain details of surface 

 sculpture, and in color. These will be considered in the following 

 pages in relation to what seem to be their principal environmental 

 antecedents. 



Ecological Conditions 



San Francisco Bay exhibits a series of conditions of peculiar 

 interest from an ecological point of view. As regards salinity, depth, 

 and temperature, and somewhat, as well, the contour of the shore, 

 the bay naturally divides itself into three major portions, which we 

 may term according to geographical position "upper," "middle," 

 and "lower." The upper bay includes San Pablo and Suisun bays 

 and the connecting straits (see map, pi. 13), being bounded on the 

 south by a line from Point San Pedro to Point San Pablo. The 

 middle bay extends from this line southward to "a line through the 



