300 University of California Publications in Zoology [Vol. 22 



finding at Goat Island, namely, the deleterious effects of crowding. 

 Our experimental timbers at Crockett were so thoroughly riddled 

 after three months that they went to pieces, and no further data were 

 obtained from them. 



It should be explained that the six months specimens were taken 

 from heavier timbers, which were not so soon destroyed. 



A few shells of known age were available from other localities, 

 and these were examined as a check on the accuracy and general 

 applicability of the above figures. From the Oakland Harbor Light 

 Station, two miles across the channel from Goat Island, 5 shells 3 

 months old averaged 16 ridges; 8 shells 4 months old averaged 18.7 

 ridges. From the Dumbarton bridge of the Southern Pacific Rail- 

 road, near the southern end of the bay, a series of 10 shells 6 months 

 old averaged 20.5 ridges. From the dock of the Angel Island Immi- 

 gration Station, a series of 10 shells of similar age averaged 20.6 

 ridges. These findings, with due allowance for the smaller numbers 

 considered, are in general agreement with the foregoing tables. 



As to the rate of deposition of ridges after 6 months, no precise 

 data are available. A series of 25 shells from Crockett, estimated 

 from the length of the burrows and the period of time the pile from 

 which they were taken was known to have been exposed to be not 

 less than 15 months old, averaged 38.8 ridges. A series of 20 shells 

 from Goat Island, judged to be of approximately the same age, aver- 

 aged 42.1 ridges. 



The largest shell which we have taken has 81 ridges. This shell 

 is from a specimen 20 inches long taken at the Oakland Southern 

 Pacific Mole, which, from its size, the thickness of the nacre lining 

 the burrow, and the badly worn condition of the pallets, may very 

 well be two years or more old. 



While we would hesitate to draw any sweeping conclusion from a 

 study of only 270 shells of approximately known age, the data 

 appear certainly to justify the statement that, under the conditions 

 of salinity and temperature occurring normally in San Francisco 

 Bay, with some exceptions later to be discussed, the shell of Teredo 

 navalis may be expected to grow at the rate of 8 or 9 ridges during 

 the first month of boring life, and from 2 to 4 ridges per month 

 thereafter. 



It will be noted that the tables show a slight but consistent advan- 

 tage in number of ridges of the shells from Goat Island over those 

 from Crockett. The significance of this fact will be pointed out 

 farther on. 



