But, as Professor Owen truly observes, 'Direct observation of the boring bivalves in the act of perforation has been rarely enjoyed, and the instruments have consequently been guessed at, or judged of from the structure of the animal.' Such, evidently, is the case with Mr. Sowerby, and several other writers who treat on this subject. Here we may call attention to the folly of naturalists endeavouring to tag a pet theory upon all the boring acephala, to the exclusion of every other. Such a system is defended upon the principle that, 'it is much more philosophical to allow that animals, so nearly allied as these in question, are more likely to effect a similar purpose by the same means, than that several should be adopted. Surely this is more consistent with the unity of the laws of nature, and that beautiful simplicity which is everywhere prevalent in her works.' How much more shrewd and philosophical are the opinions of such a man as Professor Owen, who, when speaking of the mechanical action of the valves of _P. crispata_, says, 'To deny this use of the Pholas shell, because the shell of some other rock-boring bivalves is smooth, is another sign of a narrow mind.' Again, this learned author forcibly remarks, in direct opposition to the writer previously quoted, '_The diversity of the organization of the boring molluscs plainly speaks against any one single and uniform, boring agent at all_!' The more I study this subject, the more does the truth of the last-mentioned statement become apparent to my mind. An examination of engravings of the shells, or even of the Pholas itself, when lying loose in the tank, or quietly seated in the rock, extending and retracting its siphons, fails to give one the slightest idea of its extraordinary appearance when enlarging its dwelling. At such times it seems to be a totally different animal, and to have suddenly acquired a most marvellous degree of power, energy, and perseverance, forming a striking contrast to its usual quiet, passive habits.