235 



bilities of growtli unck-r fa\'i)ral)le cciiulitions. This specimen is presumed to have 

 been 8 months old, an<l eould not possibh' have been more than S'-? nionths old, 

 as the board in which it occurred was immersed only for that period of time. Yet 

 it had attained a length of 50 cm., ha\'iiig thus grown at an a\erage rate of 6.,? cm. 

 (2^2 inches) a month over the entire period. In general, however, the rate of growth 

 of this species o\er a period ot months appears to approximate 4.7 cm. {\Js inches) 

 a month. 



^11 of the above data refer to the rate of boring in Douglas fir timber. In harder 

 woods the boring proceeds more slowly, as it does also in salinities that are much 

 below the optimum for the organisms. 



It has been frequently observed that, in the case of untreated piles standing 

 side by side, the softer, more open-grained piles are much more rapidly and thor- 

 oughly destroyed by the burrows of Teredo than tough, close-grained piles. A specimen 

 of greenheart timber submerged at the Oakland Mole, January 5, 1923, was found 

 to contain a few specimens of Teredo vavalis of a maximum length of 1.9 cm. on May 

 27, 1924. As these organisms must have settled on the wood not later than December, 

 1923, this represents the boring accomplished in 5 months or more. It has been ob- 

 served also that boring proceeds more slowly in eucalyptus piling than in fir. 



In localities in Suisun Bay, especially above Martinez, where Teredo is subjected 

 to conditions of much stress owing to the low and constantly fluctuating salinities, 

 the rate of growth of the organisms and the consequent destruction of timber are 

 much reduced as compared with localities farther down the ba\-. 



The rate of destruction of timber by the burrows of Teredo or Bankia is a func- 

 tion of the individual rate of boring and the density of the attack. These are, within 

 certain limits, \ariables with relation to one another, as growth is retarded by crowd- 

 ing. In general, however, the destruction of timber is more rapid and complete when 

 penetrated by numerous small, crowded specimens than when penetrated by fewer, 

 although larger, more rapidly boring individuals. For this reason the work of Bankia, 

 the larger and less prolific organism, is ordinarily not so immediately and phenomenally 

 destructive as that of Teredo, which is smaller, but settles on timber in much greater 

 numbers. 



A striking illustration of the possibilities of destruction of timber by Teredo 

 navalis is afforded by certain experimental timbers placed at Port Costa in 1920 by 

 the Southern Pacific Company. Rough and planed timbers of Douglas fir, 6 by 8 

 inches, were placed in 18 feet of water at the Port Costa wharf on April 2 and May 5, 

 1920. These were found on August 4 to be lightly infected (in salinities a-v-eraging 

 about 20 parts per 1000). On August 16 the infection was heavy, averaging 130 

 minute teredos per square inch of surface near the mud line. The pits had a diameter 

 of 0.008 inch, and were only deep enough to accommodate the minute shell of the 

 young borer. On August 26 the opening had increased to 0.01 inch and its depth to 

 3/32 inch, while its diameter below the surface increased aljriipth- to 1,16 inch. 

 The salinity during this period had reached 25 parts per 1000. 



On September 15 another examination was made in salinities of 28 parts per 1000. 

 I'he orifices of the burrows had enlarged to 0.013 inch, and growth had been rapid. 

 A few of the larger borers had attained a length of 1.3 to 1.8 inches, but the great 

 majority were only 0.40 to 0.65 inch. The settlement of larvae and the growth of 

 the borers continued rapidly, especially near the mud line, into October, when larval 

 settlement declined. On September 25 the penetration was 1.5 inches. The highest 

 salinity of the year, 29 parts per 1000, was recorded at this time. 



On December 8 these timbers were examined, and also some others planted 



