30 University of California Puhlications in Zoology [Vol. 18 



it is presumed that the flood tide sweeps up the bay with the least 

 reduction in salinity from the normal salinity of ocean water. On the 

 back or western side of this dike damage was noted for only about 

 2000 feet from the outer end, while on the front or eastern side the 

 damage extended for over 6000 feet shoreward from the outer end, 

 indicating that the eastern or front side of the dike was exposed to 

 water suitable for the existence of shipworms, while the water eddying 

 over the wide mudflat behind the dike was largely unsuitable for 

 them. 



In the damaged portion of the dike the operations of tlie borers 

 were more extensive in the brace piling, which extended out from the 

 sheet piling at an oblique angle, than in the sheet piling, suggesting 

 that a free circulation of water about the isolated brace timbers pro- 

 moted the greater activity of the borers in these timbers. Much of the 

 brace piling was more than half eaten through, whereas the shipworms 

 had bored hardly more than an inch below the surface of the wood in 

 the sheet piling. Further, the greatest damage in these dikes was 

 found near the mudline, suggesting that the borers throve best in the 

 denser water known to lie along the bottom of the bay. The species 

 of ship worm in this case has been identified as Teredo die g ends 

 Bartsch, by Dr. Paul Bartsch, of the United States National Museum, 

 to whom I am indebted for this courtesy. It is a small species making 

 holes averaging one-eighth or three-sixteenths inch in diameter. 



The board of naval engineers which inspected the Mare Island 

 dikes on the occasion of this damage reported that this unprecedented 

 incursion of Teredo was due to an increase in the salinity of the water 

 of that part of San Pablo Bay on account of the occurrence just 

 previously of two dry years in succession in which not only was the 

 rainfall and snowfall over the Sacramento-San Joaquin watershed 

 less than usual but in which it was so distributed that the amount of 

 runoff in proportion to the precipitation was also less than usual. 



A repetition of damage to marine woodwork in this region, due 

 to Teredo, was found in January and February, 1917, when sample 

 piles showing renewed activity of Teredo were removed from a num- 

 ber of places in the outermost dike of the Mare Island Naval Station. 

 The penetration of the Teredo in these cases was generally not over 

 three inches in depth and the extent of damage caused at this time 

 was not so great as that during the incursion of 1913-14. 



Wharf near Crockett. — Another instance of damage due to ship- 

 worms in this vicinity came to light when in the spring of 1914 a 



