THE BORIXG MOLLUSCA. 159 



clays and rocks, or shells, he finds tliat the excavating' in- 

 strument is the anterior portion of the animal, either the 

 foot and the edges of the mantle, or the edges of the mantle 

 solely. These organs arc fitted for the office they are to 

 perform not only by their position and figure, and their 

 pliability and muscular structure — made more than com- 

 monly muscular for the duty, — but also by being armed with 

 a rough layer of numerous crystalline particles of various 

 sizes and shapes, chiefly five and six-sided, and all having 

 one or more elevated points near the centre. These crystals 

 are imbedded in the surface of the boring-foot and thickened 

 edges of the mantle ; and, consisting, pro])ably, of silex or 

 flint, either pure or in combination with some animal matter, 

 they form a sort of file, — superior, however, to any of our 

 workmen's files in this, that the surface keeps itself always 

 in a proper state of roughness for trituration. This is done 

 by an organic law, which causes the crystals to be constantly 

 shed and as constantly renewed, just in the same manner as 

 the epithelial scales are on the surfaces of all exposed animal 

 membranes. 



There is in this theory, a simplicity and adequateness, 

 that seem to mark the invention as one from above ; and it 

 strongly recommends itself to our common sense. I must 

 refer you to the author's essay* for the more ample proof 

 of it, but I cannot refrain quoting some passages for your 

 immediate perusal. 



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

 and the thickened portion of the mantle of Saxicava, Gas- 

 trochaena and their allies, appear, then, to be rubbing disks 

 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 por- 

 tion 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 anterior part of 



* " On the Boring of the Mollusca into Rocks, &c. ; and on tlic Removal 

 of Poitions of their Shells." By Albany Hancock, Esq., in Ann. and 

 Mag. N. Hist. scr. 2, ii. 225. Oct. 1848. 



