233 



TABLE No. 29 



SUMMING UP TABLE 28 

 ((!, ft, f, and d omitted from averages) 



of the specimens indicated by the asterisk is generally higher than that of the others, 

 which would suggest that in wood which has been inmiersed long enough to become 

 well water-soaked boring proceeds more rapidly than in test boards only recently 

 immersed. 



It appears from the foregoing tabulation of data that the normal rate of growth 

 of Teredo navalis under favorable conditions in San Francisco Bay is 1.5 cm. during 

 the first 13^2 nionths (1.0 cm. a month), increasing to 3.5 cm. the following month, 

 and reaching a maximum of 4.7 cm. (l^s inches) during the period elapsing between 

 lYi and 3H months. After this the rate of boring declines, averaging 2.,S cm. between 

 3 J/2 and 43^2 months, and 1.8 cm. a month over the period between 4^ 2 and 14 months. 

 This decline in rate of growth during the fourth month and subsequently may repre- 

 sent a function of the normal growth cycle, or the effect of the lower temperatures 

 of winter, or both. It will be noted that during the periods September-January, 

 October-December and November-August (a, h and c in Table 28), the rate of growth 

 is considerably less than during the warmer months of the year. 



The average rate of growth per month between 14 and 24 months, in the one 

 individual considered, is 2.2 cm. per month. This slight increase over the average 

 rate between 432 and 14 months is not considered significant. It would appear that 

 the normal rate of boring over a period of nionths is in the neighborhood of 2 cm. 

 (^ inch) per month. 



The individual which reached a length of 50.8 cm. (20 inches) is the largest 

 specimen of Teredo navalis that has been found in the course of this investigation. 



The results tabulated in tables 30 and 31, although more erratic than those 

 found in the case of Teredo navalis, show a much more rapid growth for Bankia setacea. 

 They indicate that during the first 134 months the growth approximates 3.5 cm. 

 (2.3 cm. a month). During the following month the rate of growth increases slightly, 

 being 3.0 cm. Between 2^2 and 334 months the rate of growth leaps suddenly to 

 11.1 cm. As only one specimen of an age of 23^ months was available for measure- 

 ment, it is likely that the apparent rate of growth between 13^2 and 23^2 months is 

 too low, and that between 23^2 and 33'2 months correspondingly too high. But it is 

 obvious that here, as in the case of Teredo navalis, the maximum rate of growth 

 occurs during the period between 23^2 and i]/2 months. The decline which follows 

 cannot in this case be attributed to low temperatures. The breeding season of Bankia 

 is several months earlier than that of Teredo, so that this decline in rate of growth 

 occurs about m.idsummer. 



