PIIOLAS. 103 



notion that their shells are formed of calc-spar {chaux car- 

 bonate sjmthujue) seems to render impossible the perforation 

 by the shells themselves, of calcareous rocks equally hard 

 with the bodies j)erforating. But now that we know these 

 shells are composed of arragonite, it is plain that they can 

 act mechanically even on the hardest lime-stones." 



Sir Henry de la Beche has given a table of the specific 

 gravity of some shells, as observed by himself,* in which 

 he states that ofPholas crispata to be 2"82, and of a number 

 of other bivalves and univalves to be nearly the same ; 

 indicating a constitution which, allowance being made for 

 amount of animal matter, approximates their shells much 

 more nearly to arragonite than to calc-spar, the former hav- 

 ing a specific gravity of 2"93, and the latter of 2"7l."}' 



Mr. Albany Hancock;j: has jrat forward one of the most 

 definite and important opinions ever expressed on this dis- 

 puted question. He states that — " The excavating instru- 

 ment of Pholas and Teredo is formed of the anterior por- 

 tion of the animal, in the surface of which are imbedded 

 siliceous particles. The particles penetrating the skin, give 

 to it much the character of rasping paper. The whole 

 forms a rubbing surface, which being applied closely to the 

 bottom of the cavity by the adhesion of the foot, enables 

 the animal to rub down, and so penetrate, shale, chalk, 

 wood, or even the hardest lime-stones and marble. 



" Saxicava rugosa is also furnished with a rasping sur- 

 face covered with silicious particles. This surface, how- 

 ever, in this species, is formed entirely of the anterior por- 

 tion of the mantle, the margins of which being united, are 



* Theoretical Researches, p. 75. 



t Mr. Trenham Reeks has hitely examined the specific gravity of several species 

 of I'/iolas, and has kindly communicated the results : he finds that of P. crisjxita 

 to be 2"7G ; /'. ductylus, 2'45 ; P. cimdida, 2'70 ; and P/ioladidea jmpyracea 

 2-64. t An. Nat. Plist. vol. xv. p. 114. 



