Because, being found in abundance, in a great variety of rocks and positions, they constitute the medals of the ancient world; and, from an accurate acquaintance with their different species, and with the nature of the animals that represented them, many curious and important deductions respecting the formation and changes of the crust of the earth may be drawn. - Thomson's Hist. Royal Soc.
Testacea, (or shell animals) are classed in families, according to the number and form of the shells. Thus multivalve, or with many valves; bivalve, muscles, etc.; univalve, with regular windings, snails; and univalves, without such windings.
Because of their service in the domestic economy of various nations. Thus, in the south of China and in India, the thin layers of some large flat shells, when polished, are used instead of glass for windows. Many of the domestic utensils of savage people are shells; and it must have been observed, that we frequently imitate these in our porcelain. In India, drinking-cups are formed of the nautilus; and in other less civilized nations, shells are converted into knives, spoons, fishing-hooks, razors, etc. In Zetland, a certain shell, suspended horizontally by a cord, is used as a lamp, the canal serving to hold the wick, and the cavity to contain the oil. Is it not probable that some of the most elegant patterns left us by the Greeks have been suggested by a similar primitive practice.
In mentioning the benefits of shells, we must not forget the celebrated Tyrian purple, which was procured from a univalve shell fish, and was contained in a transparent and branching vessel, placed behind the neck of the animal. A shell, called Purpura, of our own shores, furnishes a liquid of similar qualities, and is supposed to have been resorted to by the ancients.
Because, in the days when Ossian sang, the hollow shells of the scallop were the drinking-cups of Fingal and his heroes, and the flat shells their plates. Thus, in Ossian: " Thou, too, hast often accompanied my voice in Branno's hall of shells." " The joy of the shell went round, and the aged hero gave the fair."
Because they consist of layers of an earthy salt, with interposed membranes of animal matter, resembling coagulated albumen, and they grow, by the addition of layers of new matter, to the edges and internal surface. When broken, the animal can cement the edges, and fill up the crack, or supply the deficiency when a portion is abstracted. - Fleming.
As an instance of the great strength of such cement, it was, in 1829, stated in Branded Journal, that the large snails which are found in gardens and woods, discharge a whitish substance, with a slimy and gelatinous appearance, which has been known to cement two pieces of Hint so strongly, as to bear dashing on a pavement without the junction being disturbed, although the flint broke into fragments by fresh fractures.
Because the colour is secreted by the animal, along with the matter of the shell.
Because the animals, to form this inner layer, excrete the lime in nearly a pure state, that is to say, mixed with much less animal matter; so that, in concreting, the particles become very close and compact, and receive a polish from the repeated frictions of the soft parts.
Because, being a dried sheet of coagulated albumen effused at the same time, or, perhaps, even prior to the first layer, it may protect the subjacent or more chalky layers from the action of the air or water during their consolidation. - Mr.Gray, in the ZoologicalJourn.
Blainville, and the French naturalists, generally, suppose that the above covering is the true epidermis, or scurf-skin, raised from its position by the deposition of the shell underneath it; but we think this is not tenable.
Because a shell having attained its full growth, the changes which the animal further effects are almost limited to some increase of its thickness; not, however, by the addition of any new layers, but by the effusion of vitreous matter. Hence, holes and canals, previously visible, are now filled up; the aperture contracted, and the margins strengthened and enlarged; the upper part of the spire, perhaps, filled and made more solid. The external layers now lose their epidermis; the colours become paler; parasites deform and perforate the outer surface ; death at length overtakes the architect, and the shell decays under the influences of the water and the air. What an epitome is this of the proudest life, - till death Comes at the last, and with a little pin Bores through his castle-wall, and - farewell king.