Page:Natural History, Mollusca.djvu/319

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307
BORERS.

that of Pholadidea have been chemically examined by our friend, Mr. Trenham Reeks, with a negative result as regards the presence of particles of silex in their substance, where, after the statement of Mr. Hancock respecting the structure of the mantle, we thought they might possibly be found. On the other hand, taking into consideration its mineralogical nature, as stated by M. Necker, there is no reason for supposing that the shell of the Pholadidæ is so weak a perforating instrument as some have fancied. With its peculiar form, and the saw-like asperities of its surface, especially of its anteal extremity, it is well adapted for an auger, when wielded fresh and elastic by its well-muscled animal inhabitant, whose foot, in all the members of this tribe, even in Teredo, where it is least developed, seems specially organized to serve as a fulcrum. We have no evidence that they perforate any substances essentially harder than their shells, or so hard. The sandstones in which they occasionally occur are either friable or marly, when fresh, though cabinet specimens seem so solid. The explanation of Necker accounts for their perforations in the hardest limestones. Wood, wax, and other substances in which they occur, offer no difficulty. The statements put forward respecting their boring in lava and granite, have long ago been shown to be mistakes. That they exhibit a rotatory motion, during the action of boring, has been proved by competent observers; and the cavities they excavate, if examined when fresh, invariably show transverse groovings, which could have been caused only by such motions. Currents of water, set in motion by cilia, doubtless aid materially the animal's operations, and possibly may