coarser in construction and quality as they approach the outer surface.
In
the inside of the shell, the calcium carbonate plates are very fine
and transparent, and the animal membrane in which they are set is of
extreme tenuity. In the middle shell these plates become more chalky
and less compact; in the exterior shell they are set in a thicker
binding of organic matter and terminate outside in rough, horny
fringes, completely covering the shell.
In
a general way therefore, the animal deposits the best of its secretions
about itself and pushes out to the outer extremities, the coarser
elements which are fitted to preserve the finer parts of the shell, as
the finer parts of the shell are fitted to protect the delicate
organism which they enclose. The building of the shell is done by a
membraneous covering of the fish which entirely envelops the body and
is attached to the shell a short distance from the inner edge, leaving
a rim of membrane free around the fish and the edges of the two valves.
This membrane is called the mantle. It extracts lime from the water,
and at different parts exudes modified solutions of it mixed with
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