The mode in which the boring Mollusca execute their perforations is yet a matter of uncertainty. The principal hypotheses that have been put forth on the subject have been already briefly enumerated, but it may not be amiss to add an expression of opinion on their value, and a careful summing up of the evidence, by zoologists eminently worthy of deference, the able authors of the "British Mollusca."
PHOLAS."Of all these theories," observe Messrs. Forbes and Hanley, "the chemical one, so far as a secreted solvent is concerned, bears least examination in the case of the Pholadidæ. The substances perforated are wood, limestones, hard and soft, argillaceous shales, clays, sandstone, and (in the case of a Pholas, in the magnificent collection of Mr. Cuming) wax. The notion of a secreted solvent, that would act indifferently on all these substances is, at present at least, purely hypothetical; and, since all attempted tests have failed to detect an acid,
