304 



2 feet above mean low water to 1 foot below, which was the lowest level at which the 

 observers were able to work. It doubtless similarly occurs on down to the mud, e\en 

 in deep water, as it has been dredged in San Francisco Bay at a depth of 50 fathoms. 



This borer, unlike Teredo, has the body entirely enclosed within the two valves 

 of the shell, which are ovate, tapering somewhat posteriorly, and ending in leathery 

 flaps. During the period of active boring life the foot protrudes through a rather 

 large anterior gap between the valves; but after cessation of boring, and perhaps in 

 the interim between periods of boring activity, this gap is closed over by a calcareous 

 plate, giving the borer the appearance shown in the photographs (figure 120, 1 and 4). 

 It will be noted that the anterior portion of the shell is ribbed and somewhat denticu- 

 lated, as for rasping, or to grip the sides of the burrow. 



This species is edible and is used for food in localities where it occurs in sufficient 

 abundance to justify the labor of removing it from its rocky domicile, which is usually 

 done by means of iron bars. 



Another boring species found in these jackets was Platyodon cancellala Conrad 

 (fig. 120, 2), a near relative of the soft-shell clam (Mya arenaria). This borer normally 

 inhabits stiff mud and clay. It ^\•as found in the pile jackets only sparsely, and in 

 decidedly poor mortar. 



A third species occurring in the jackets was the so-called "nestler," Petricola 

 carditoides Conrad (fig. 120, 3), which is believed not to bore on its own account, but 

 to inhabit natural cavities or holes bored by other organisms. 



As regards the extent of the damage occasioned by the rock borers, a review of 

 the data assembled indicates that, of concrete jacketed piles at four different locations 

 in Los Angeles Harbor, in fact, at every point in the inner harbor where such piles 

 exist, about 50 per cent have been more or less attacked, of which rather more than 

 one-fifth have been very considerably bored. Of those not attacked, a number stood 

 so well inshore as to be but little exposed to the action of the borers. If all such piles 

 were eliminated from the count, the percentage of jackets damaged would be con- 

 siderably higher. 



These jackets were in general of cement mortar poured around the piles by setting 

 forms after the piles were driven. Some of the jackets had given service in sea water 

 over a period of 14 years; but they would be regarded as decidedly inferior according 

 to present day standards as specified elsewhere in this report. Nevertheless some of 

 them were of considerable hardness. 



Mr. A. A. M. Russell, Testing Engineer for the State Harbor Commission, made 

 crushing tests of two of the best samples of mortar in which borers were found. One 

 of these, a specimen 2^2" x Ti}^", i'^v" high, had a crushing strength of 1726 pounds 

 per square inch. This sample when crushed showed an encased sand pocket from 

 which the aggregate could readily be picked with the fingers. Mr. Russell reports the 

 grading of the aggregate as follows: 



Screen Percentages 



10-20 1 .78 



20-30 .89 



30-50 4.46 



50-80 51.78 



80-100 16.07 



100-200 16.07 



Pass 200 8.94 



Approximately 93 per cent of the total aggregate passed the 50 mesh screen. 



