298 University of California Publications in Zoology [Vol. 22 



Series of specimens of the ages indicated were available for 

 comparative stud}' from several localities in San Francisco Bay. The 

 first group to be discussed is that from the lighthouse depot at Goat 

 Island, in the central portion of San Francisco Bay. The ridges on 

 the anterior lobe of the shell were counted under a magnification of 

 35 diameters. The results are given in table 1. 



A source of error lies in the fact that, while new ridges are being 

 added at the margin of the shell, certain of the earlier ones are lost 

 by erosion or by the overgrowth of shelly material mentioned above, 

 so that the figures in the table may represent a few less than the 

 actual total of ridges deposited in a given time. This error seems 

 unavoidable, and is negligible for practical purposes. 



Data from older shells of known age are not available from this 

 point, owing to the fact that our test timbers were greatly weakened 

 at the end of the fourth month, and a heavy storm in December 

 entirely destroyed them. 



It will be noted that the rate of growth, as measured by addition 

 of ridges, is extremely rapid during the first month (9.3), after 

 which it drops suddenly and remains nearly constant during the next 

 two months (3.8 and 3.6). This is what we should have expected 

 from observing that the burrows of Teredo expand rapidly in the 

 first inch or two of their length, after which the increase in diameter 

 is slow and nearly constant. During the fourth month a further 

 decrease (2.6) in the rate of growth is evident; this may indicate 

 the influence of lowered temperature (the fourth month ended De- 

 cember 15) or the effect of crowding, as the timbers were already 

 entirety honeycombed, so that further growth was much limited, if 

 not indeed impossible. 



At Goat Island we have a condition of high and fairly constant 

 salinity, considerable depth (26 feet at low tide), and a minimum 

 range of temperature fluctuation. Fortunately, we were able to 

 make a comparative study (table 2) on material available from 

 Crockett, a locality well toward the upper reaches of the bay. At 

 this station the water is brackish and the temperatures more variable, 

 owing to the inflow of a considerable volume of water from the San 

 Joaquin and Sacramento rivers. The depth here is about the same 

 as at Goat Island. Only specimens of the ages indicated were avail- 

 able. 



Here again we note a rapid growth during the first month of 

 boring life, followed by a sudden decrease, the figures for the first 



