* The gaudy conch. l. 66. The spiral form of many shells seem to have afforded a more frugal manner of covering the long tail of the fish with calcareous armour; since a single thin partition between the adjoining circles of the fish was sufficient to defend both sur∣faces, and thus much cretaceous matter is saved; and it is probable that from this spiral form they are better enabled to feel the vibrations of the element in which they exist. See note on Canto IV. l. 162. This cretaceous matter is formed by a mucous secretion from the skin of the fish, as is seen in crab-fish, and others which annually cast their shells, and is at first a soft mucous covering, (like that of a hen's egg, when it is laid a day or two too soon,) and which gradually hardens. This may also be seen in common shell snails, if a part of their shell be broken it becomes repaired in a similar manner with mucus, which by degrees hardens into shell.
It it probable the calculi or stones found in other animals may have a similar origin, as they are formed on mucous membranes, as those of the kidney and bladder, chalk-stones in the gout, and gall-stones; and are probably owing to the inflammation of the membrane where they are produced, and vary according to the degree of inflammation of the membrane which forms them, and the kind of mucous which it naturally produces. Thus the shelly matter of different shell-fish differs, from the courser kinds which form the shells of crabs, to the finer kinds which produces the mother-pearl.
The beautiful colours of some shells originate from the thinness of the laminae of which they consist, rather than to any colouring matter, as is seen in mother-pearl, which reflects different colours according to the obliquity of the light which falls on it. The beautiful prismatic colours seen on the Labrodore stone are owing to a similar cause, viz. the thinness of the laminae of which it consists, and has probably been formed from mother-pearl shells.
It is curious that some of the most common fossil shells are not now known in their recent state, as the cornua ammonis; and on the contrary, many shells which are very plentiful in their recent state, as limpets, sea-ears, volutes, cowries, are very rarely found fossil. Da Costa's Conchology, p. 163. Were all the ammoniae destroyed when the continents were raised? Or do some genera of animals perish by the increasing power of their enemies? Or do they still reside at inaccessible depths in the sea? Or do some animals change their forms gradually and become new genera?
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