For hundreds of years this query has been asked, and various are the replies which from time to time have been given. Singular to state, although specimens of the Pholas, and its allies the Saxicavæ, are to be procured in abundance in many parts of the kingdom, the subject is not even yet positively settled. There have been many theories advanced, some the result of fancy or guess-work; others, of practical study. All these have their supporters, but none have, by common consent, been adopted by physiologists as the true one. Having for several years made this subject a study of personal observation, I believe I may venture to state, that I have succeeded in casting a feeble ray of light upon it; and, although the result of my labours may not be deemed conclusive, I may at least claim some credit for my endeavours to clear up a most difficult, though deeply interesting point in natural history. The various theories promulgated on this knotty point are generally classed under five heads: 1st, That the animal secretes a chemical solvent--an acid--which dissolves the substance in which it bores. 2d, That the combined action of the secreted solvent, and rasping by the valves, effects the perforations. 3d, That the holes are made by rasping effected by silicious particles studding the substance of certain parts of the animal. 4th, That currents of water, set in action by the motions of vibratile cilia, are the agents. 5th, and lastly, That the boring mollusca perforate by means of the rotation of their shells, which serve as augurs. Of all the above, the first which is quite a fancy theory, seems to meet with greatest favour among certain naturalists. But as it is rather puzzling to find a chemical solvent, which will act equally upon sandstone, clay, chalk, wax, and wood, this hypothesis can only be looked upon by practical men as ingenious, but incorrect. Even were it proved that the animal really possessed the power of secreting an acid sufficiently powerful, the question naturally arises, How can the shell escape being affected in like manner with the much harder substance in which it is situated? The second theory, or the combined action of rasping and the secreted solvent, is, for obvious reasons, equally objectionable. The third theory, which endeavours to account for the wearing away of the rock by means of silicious particles situated in the foot and other parts of the animal, has been for some time proved to be erroneous, from the fact, that the combined skill of some of our best anatomists and microscopists has failed to discover the slightest presence of any particles of silex in the Pholadidæ, although these are believed to exist in other families of the boring acephala. The fourth theory, that of ciliary currents as an accessory agent in boring, is worthy of greater consideration, chiefly from the evidence we possess of the immense power which the incessant action of currents of water possess in wearing away hard substances.