J! '- 2 ] Miller: Variations in Teredo navalis in Sun Francisco Bay 315 



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 pigmented and more translucent, and 

 in general appearance give the impression of intermediateness be- 

 tween 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 differ appreciably 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 environmental 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 Straits taken in 

 1920 differed appreciably from those taken in 1921 and subsequently, 

 being distinctly more like the shells typical of the middle bay. For 

 example, 40 shells collected at Crockett in 1920 averaged 15.8 ridges 

 per millimeter on the anterior median portion, as compared with an 

 average of 14.5 for shells collected there in 1921 and later. This 

 proved very puzzling, until it was remembered that the winter of 

 1919-20 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 salinity in this region (Kofoid, 

 1921, p. 49). With the resumption of normal river discharge in the 

 winter of 1920-21 the salinity was lowered. This seems to be the 

 explanation of the differences found between shells collected here in 

 1920 and 1921 respectively. 



The winter of 1919-20 was marked by probably the lowest river 

 discharge since 1863-64 (Grunsky, 1921, p. 15) ; shells collected 

 during this period were accordingly excluded in preparing the fore- 

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



