40 University of California Publications in Zoology [Vol.18 



17.27 to 19.25 parts per 1000. It seems probable, therefore, that 

 Teredo dicgensis, once established, can withstand for several weeks or 

 perhaps months a reduction of salinity to about 10 parts per 1000, 

 though perhaps thriving sufficiently to cause marked damage only 

 in a region where the salinity of the water, except for a short period, 

 must average at least 13 or 14 parts per 1000. 



In common with a recognized biological principle, the adult ship- 

 worms, moreover, may be expected to be hardier and more resistant 

 to adverse circumstances than the larvae. Thus the larvae might be 

 able to invade San Pablo Bay only in the period of maximum salinity 

 in the bay during the late summer and fall, but once established and 

 developed into adult animals the shipworms might much better with- 

 stand the reduction in salinity of succeeding freshets of the winter and 

 spring, if tliis reduction should not fall below a certain minimum limit. 



It is possible also that Teredo diegensis may breed normally in 

 localities where the salinity is lower than that which Xylotrya sctacea 

 can endure, and that damage to unprotected marine woodwork will 

 occur in such localities of reduced salinity from increased numbers of 

 native Teredo diegensis when, as occasionally happens in San Pablo 

 Bay, a "dry" year permits the salinity to rise above the condition 

 usually prevailing. In this connection a more extensive knowledge 

 of the distribution of Teredo diege7ms and of Xylotrya, setacea in 

 such a region as San Francisco Bay than is at present available is to 

 be desired. 



From the known rate of operation of shipworms of the small size 

 of those found in San Pablo Bay in other parts of San Francisco Bay, 

 it seems hardly probable that the extensive damage to the brace piling 

 of the Mare Island dikes, in which many of the timbers were from 

 one-half to three-quarters destroyed, could have taken place during 

 the six or eight months following the falling off of the discharge of the 

 Sacramento and San Joaquin rivers in the summer of 1913. The 

 conclusion seems justified, therefore, that the damage reported in 

 January, 1914, dates back to an infection during the summer or fall 

 of 1912, at least a year and a half previously, but hardly to the fall 

 of 1911, two years and a half before. 



The general observation of engineers in San Francisco Bay is that 

 the shipworm is much more active at or near the mudline than near 

 the tide levels. This same difference in the level of the greatest activity 

 of these borers was noted in the dikes of the Mare Island Navy Yard, 

 also in the piles extracted from the wharf of the Union Oil Company 



