gratuitously so; for we can hardly suppose that any of those who have taken this view of the cause would maintain, that the animals have the power of secreting different acids at will, according to the substance they have to attack. Yet this notion has been most favoured by naturalists, who, sceptical as to the perforating power of such fragile instruments as are the shells of many of these creatures, endowed the animals with supernatural chemical qualifications. Even good experimental observers—Mr. Osler for one—whilst they proved that the Pholas could bore mechanically by the rotation of its valves, could not free their minds from the prejudice in favour of a solvent. The important statement put forward by Mr. Albany Hancock, respecting the instruments by which Mollusca bore, and which, so far as Gasteropoda are concerned, appear to furnish us with a true explanation,—namely, that it was effected by means of silicious particles, variously arranged in certain portions of the animal's body,—led us to hope that a better cause than any yet alleged had been discovered. But we cannot bear it out with respect to the Pholadidæ. We can find no such particles in the mantle of the Teredo, nor have any been noticed by Home or Deshayes, or by the most recent observers, Fray and Leuckart, who paid especial attention to the structure of the tissues in this genus. Nor could we, although aided by the anatomical and microscopical skill of Mr. Busk, detect any silicious particles in either the mantle, foot, or siphon tube of Pholas candida. If present in any species, therefore, they are exceptional, so far as the genus Pholas and its allies are concerned. The shells of several British species of Pholas, and
