289 



organism has succeeded in burying its shells ironi sight. Thus the immediate correlation 

 between the boring habit and the structure of the shell is illustrated. 



Further growth of the shell occurs liy addition of successive increments to the 

 margin in such a way that the newly established shape of the shell is but little altered. 

 The rate of addition of new ridges is most rapid during the first month of life in the 

 wood, and decreases progressively thereafter, although growth probably does not 

 entirely cease until the organism is near death. By our system of planting and removing 

 test timbers at regular intervals in infested localities, it has been possible to get a 

 limited nimiber of shells whose age can be estimated with a considerable degree of 

 accurac\-. The following data were obtained l)y averaging the number of ridges that 

 could be counted on 25 shells of each age. A source of error lies in the fact that, while 

 new ridges are being added at the margin of the shell, certain of the older ones are 

 lost by erosion or by a secondary overgrowth of shell substance, so that the figures 

 given 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, since 

 we desire principally to know what age a given number of visible ridges indicates. 



Data from older shells of known age have not, in general, been available, owing 

 to the fact that our test timbers have been, at the end of irom four to six months, 

 entirely destroyed, or so weakened that they were washed out by tidal currents or 

 wave action. A series of 25 shells from Crockett, estimated from the length of the 

 burrows and the period of time the pile from which they w'ere taken was known to 

 have been exposed, to be not less than 15 nn)nths old, axeragcd 38.8 ridges. A series 

 of 20 shells from Goat Island, judged to be of approximately the same age, averaged 

 42.1 ridges. 



TABLE No. 43 



RATE OF GROWTH OF RIDGES ON SHELLS OF TEREDO XT GO.VL ISL.\.\D 



TABLE No. 44 

 RATE OF t;RO\VTH OF RIDGES ON SHELLS OF TEREDO AT CROCKETT 



