242 Mr. A. Hancock on the Boring of the Mollusca into Rocks, 



these bodies^ and when we refer to the fact recently made known 

 that the spines of the tongue of the Gasteropods are composed of 

 silex, a high degree of probabiUty is estabhshed in favour of this 

 view ; and if it be correct, the phsenomena attending the boring 

 of the moUusks are very easily explained*. 



The foot and mantle of Teredo, Pholas and Patella, and the 

 thickened portion of the mantle of Saxicava, Gastrochana and 

 their allies, appear, then, to be rubbing discs of extraordinary 

 power, crowded as they are with these siliceous bodies, which 

 penetrating the surface give to it much the character of rasping 

 or glass-paper. And all that now remains to be proved is the 

 existence of muscles to give to this formidable cutting surface 

 the necessary rubbing motion. 



These muscles are amply provided : the adhesive portion of 

 the foot, as well as the mantle, of Teredo and Pholas, and also 

 of Patella, are composed of interlaced muscles. The anterior 

 thickened part of the mantle of Saxicava is also made up of 

 muscular fibres running in all directions. And Professor Owen, 

 in his account of Clavagella, states that " the muscular layer 

 after forming the siphon and its retractors is confined to the an- 

 terior part of the mantle, where it swells into a thick convex mass 

 of interlaced and chiefly transverse muscles." Surely this powerful 

 muscular apparatus has some important function to perform : — 

 not to secrete a solvent, but to assist by its mechanical agency 

 in the work of excavation. 



We now see the boring instrument complete in all its parts ; 

 and a more efficient apparatus could not be devised. Supplied 

 with this flinty armature, the soft fleshy foot of Pholas and Teredo, 

 adhering to the substance to be reduced, and aided by the edges 

 of the mantle, cuts with equal facility into wood, shale, chalk, 

 and the various other bodies into which these mollusks burrow. 

 Patella excavates in the same way. The mode is somewhat 

 varied in Gastrochcena and Saxicava : they firmly attach them- 

 selves by the byssus to the rock, then bring into contact with it 

 the armed and thickened portion of the mantle ; thus enabling 

 the interlaced muscles of which it is composed to work with as 

 much efi*ect as those in the broad> adhesive foot and mantle of 

 Pholas and Teredo. 



In none of these species is much rotatory motion required. In 



* In the * History of British Mollusca ' the existence of siliceous bodies 

 in the foot and mantle of Pholas and Teredo is denied. Perhaps the authors 

 of that work may have overlooked them on account of their resemblance to 

 epithelium scales. Silex, however, can scarcely be considered essentially 

 necessary ; a much softer material on a living surface, and perpetually being 

 renewed^ may be supposed capable of rubbing down the various substances 

 into which these animals burrow ; certainly so far as the Pholades and Tere- 

 dines are concerned. 



