18 

 LITHOPHAGA. 



Boring shells, without accessary pieees or sheath, and' 

 more or less gaping at the anterior side* Ligament external. 



SAXICAVA. 

 GENERIC CHARACTER: Shell bivalve, transverse, the 

 sides unequal; gaping interiorly at the superior margin; 

 hinge almost without teeth. 

 * S. RUGOSA. Mytilus R. Turt. Lin. Pen. Brit. Zo M 

 vol. 4, pi. 63, fig. 72. Mag. Nat. Hist., O. S., vol 4, 

 p, 358. The boring species of shell-fish found in Britain 

 belong chief!}' to the genera Teredo, Pholas and Saxicava ; 

 for it may be doubted whether any of the other families, 

 though found in similar situations, are able to form for 

 themselves the chambers they inhabit. The Teredines are 

 found only in wood, which when soaked in sea-water they 

 readily devour, so that their intestines are found greatly 

 distended with it. The Pholades prefer the hard rock : 

 either the common slate of our coast, which when young 

 they readily penetrate, and hollow smoothly out to the 

 depth of several inches, with a diameter equal to the com- 

 fortable expansion of the shell : or the sandstone found in 

 the sea at no great distance. Limestone is also subject to 

 their depredations, but it seems, less frequently ; and when 

 the chamber is become sufficiently large for their full 

 giowth, no further destruction ensues, except from the mul- 

 tiplication of individuals. The Saxicava Rugosa, though a 

 much smaller Animal, is far more destructive, from being 

 much more generally destributed, and in greater numbers. 

 It has also of late attracted special attention from having 

 spread its ravages along the whole front of the Breakwater 

 at Plymouth; and thereby excited in the minds of some, 

 very serious apprehensions for the safety of that edifice, 

 Its devastation appears to be confined to substances of 

 which a chief constituent is lime; which it enters when 

 young, and which it is never found to penetrate beyond the 

 depth of about six inches. It can therefore only be when 

 the honey-comb-work of its operation shall have been 

 broken of by the violence of the waves, that a further ex- 

 tension of its ravages can be apprehended : a circumstance 

 which may be prevented by occasionally throwing along 

 the face of the structure, fragments of mineral substances, 

 as iron or copper ; the poison of which would be fatal to 

 the Animals. Their operation is not observed to extend 

 much below the low water mark. The manner in which 

 these burrowing Mollusks effect their operation on the 

 solid rock, has been the subject of controversy, and 

 perhaps may differ in the different families. It is certain 

 that the Teredines effect the destruction of wood by the 



