306 University of California Publications in Zoology [Vol.22 



study specimens of Teredo from localities most distinctly character- 

 ized by the physical differences we have described. Specimens from 

 the upper bay have been taken principally from the region of Car- 

 quinez Straits, those representing the middle bay from Goat Island 

 aud from the Southern Pacific and Western Pacific moles, and those 

 considered typical of the lower bay have been collected at Dumbarton. 



Variation in Number of Ridges 



It has already been pointed out, in discussing the growth of the 

 shell, that shells from Goat Island show a slight but consistent advan- 

 tage in number of ridges over shells of the same age from Crockett. 

 The significance of this was not immediately evident. In order to 

 let further light on the matter, it was decided to count on several 

 series of shells the, number of ridges per millimeter on an arbitrarily 

 chosen part, in order to determine whether or not there is actually 

 a greater number of ridges per unit area on the shells from the 

 middle bay. The portion of the shell chosen was the first millimeter 

 of the anterior median denticulated area, approximately in a line 

 with the lower edge of the anterior lobe, but definitely at right angles 

 to the ridges. The following curves (Fig. D) are based on 50 shells 

 from each of three localities ; the shells have been selected at random, 

 but are not the same ones used in preparing the above tables of rate 

 of growth, which makes the correspondence in results the more sig- 

 nificant. 



These curves appear to indicate a very definite correlation between 

 environmental conditions and the number of ridges per unit area on 

 the shell, the greatest number of ridges being found on shells from 

 the middle bay, and the least number on shells from the upper bay, 

 while those from the lower bay stand intermediate between them. 

 The exact reason for this correlation is not entirely clear. We would 

 suggest that it lies both in the absolute differences in temperature 

 and salinity and in the range of such differences. 



That environmental conditions play a prominent part in deter- 

 mining the deposition of ridges there can be no doubt. We are often 

 able to read in the sculpture of a series of shells from a given locality 

 the history of some definite alteration in the life conditions, which 

 has left its mark on a number of shells. As an example of this, note 

 in plate 14 the unusually wide interspace by which the newest ridge 

 is separated from its neighbors, this same condition occurring in a 

 number of shells taken at the same time and place. 



