circles of the fish was sufficient to defend both surfaces, 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 is 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.