283 



are ratlu-r marked, the mean dirtereiues aiiKing shells e\en from these specially 

 selected localities are comparatively slii^ht. In many, indeed in most, cases it is im- 

 possible to take an isolated unmarked shell and declare from what localitN' it has 

 come. By referring again to figure 106 it will be seen that shells having from 1,S to 

 18 ridges per millimeter might come from any of the three localities considered, the 

 curves overlapping to this extent. From figure 111 it is evident that the index of the 

 auricle may range from 30 to 70 without indicating the locality from which a shell 

 has come. A similar statement may be made regarding the other points of difl^erence 

 noted among these three environmental races. It is only in the case of the extreme 

 variates that a locality diagnosis can be made on the basis of intrinsic characters of 

 the shell. 



Furthermore, as we depart from the extremes of environment, the differences' 

 mentioned appear to become more and more obscure, until they are entirely lost, the 

 various races merging with each other at intermediate points. Sufficient material is 

 not available adequately to illustrate this, but a few specimens are at hand from 

 stations between Crockett and Goat Island which indicate at least the trend to such 

 coalescence. F"rom Oleum (see map) we find in 25 shells a variation of from 14 to 

 20 ridges per millimeter on the anterior median area, and a range of from 28 to 67 in the 

 index of the auricle, thus indicating, with due allowance for meager numbers, a slight 

 departure in both these respects from the shells from Crockett, and a tendency more 

 nearly to resemble shells from Goat Island. The shells from Oleum are also less pig- 

 mented and more translucent, and in general appearance give the impression of 

 intermediateness between the brackish water and the salt water types. They differ 

 from the Dumbarton shells in having rather more prominent lines of growth. 



A solitary specimen from Pinole in our collection cannot be said to dift'er appre- 

 ciably from the specimens from Oleum. 



On the other hand, we have from off Point San Pablo and Point Richmond a few 

 immature specimens which closely resemble Teredo of similar age from Goat Island. 



These data, fragmentary as they are, throw much doubt on the advisability of 

 attaching varietal names to any local groups. The free swimming larvae of Teredo 

 are swept about by shifting currents to every portion of the bay, and with every 

 breeding season a new distributional assortment must occur, so that any locality 

 differences which appear in the mature animals must be the immediate result of 

 en\'ironmental factors acting separately on each generation. The most conclusive 

 proof of this has come in a very interesting way from what at first appeared to be a 

 discrepancy in our data. It was observed that shells from the region of Carquinez Strait 

 taken in 1920 differed ajipreciably from those taken in 1921 and subsequently, being 

 distinctly more like the shells typical of the middle bay. For example, 40 shells col- 

 lected at Crockett in 1920 averaged 15.8 ridges per millimeter on the anterior median 

 portion, as compared with an a\'erage of 14.5 for shells collected there in 1921 and 

 later. This prcned \er\' puzzling, until it was remembered that the winter of 1919- 

 1920 was a period of unusually scant rainfall and consequently lessened river discharge 

 into the northern arm of the bay, resulting in a protracted period of rather high salin- 

 it\' in this region (Kofoid, 1921, p. 49), With the resumption of normal river discharge 

 in the winter of 1920-1921 the salinity was lowered. This seems to be the explanation 

 of the differences toimd between shells collected here in 1920 and 1921 respectively. 



The winter of 1919-1920 was marked by probably the lowest river discharge since 

 1863-1864; shells collected during this period were accordingly excluded in preparing 

 the foregoing tables and graphs, as representing a departure from the normal. They 



