construction, i.e., separable
plates composed of thinner plates more compacted together, and these in
turn of infinitesimal hexagons of calcium carbonate; full plates,
component plates, and particles, all alike surrounded by animal tissue.
The
shell is built up of secretions from the water in which the oyster
lives, made by the mantle, a membraneous covering of the fish. The
function of this mantle, in part, is to obtain from the water the
elements required and exude it at different parts of its folds in the
various forms required for the several parts of the shell. The
necessary lime exists in the surrounding water and is supplied
sometimes by the calcareous beds upon which the oysters grow, and in
other cases by surrounding vegetation.
In
all mother-of-pearl oysters and the freshwater mussel -unio, the
lining is usually quite thick, but in some pearl-bearing species having
small, frail shells, it is, though beautiful, too thin to be of use. In
the meleagrina, this nacreous lining lies in the interior of the shell
like a congealed pearl wave, the smooth even
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