the same ingredients and are constructed on the same general principles by the animals inhabiting them.
This
description of pearl shells has been given here because a knowledge of
the shell enables one to understand the formation and characteristics
of a true pearl, and the differences which exist between the gem and
other similar formations formed in pearl and other oysters, mussels,
and univalves. Many such formations are found, having the elements and
constructed like one or both of the outer parts of the shell, and some,
in part like the lining, but these are not true pearls; the gem has
neither the material nor construction of the middle and outer shell.
Except that the pearl, because of its form, is rarely iridescent even
to a slight degree, whereas the nacreous lining of some pearl-bearing
shells is brilliantly so, the pearl and the nacre of the shell in which
it grows, are essentially the same. Pearls are more or less spherical
and independent formations, made by the fish on the same plan and from
the same secretions with which it lines the shell, misdirected by
abnormal conditions. Those con-
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