depth of four inches; and it is observed that the nodules of quartz and feldspar, which sometimes occur in the hard limestone of the pillars, are untouched.
Many theories have been invented to account for the singular power exercised by these animals, such as the following; that the animals entered the rock while it was in a soft and plastic state, and that it afterwards hardened around them—that the animal poured out some peculiar fluid which had the chemical property of dissolving the rock—that the latter was ground away by the roughnesses on the shells as they revolved, as if by the action of a rasp or file—that the minute particles of the stone were one by one separated and driven off by the force of currents of water, produced by vibrating cilia: but all these theories appear to be set aside by the discovery of Mr. Albany Hancock, one of the highest living authorities on the subject. This gentleman finds that the excavating instrument is the anterior portion of the animal, either the foot and the edges of the mantle, or the edges of the mantle solely. These organs are fitted for the office they are to perform, not only by their position and figure, and their pliability and muscular structure,—made more than commonly muscular for the duty,—but also by being armed with a rough layer of numerous crystalline particles of various sizes and shapes, chiefly five- and six-sided, and all having one or more elevated points near the centre. These crystals are imbedded in the surface of the boring foot and thickened edges of the mantle; and, consisting, probably, of silex or flint, either pure or in combination with some animal matter, they form a sort of file,—superior, however, to any
