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THE PEARL
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ciently elongated to give the pearl the obovoid form.
Many
pearls are shaped like a capsule. The ends of most are rounded up to a
full dome; some have somewhat flatter ends; many are long and
cylindrical like an ordinary capsule; others are short and appear in
shape like two high button pearls joined at their bases; while some
resemble a cartridge, one end being almost flat and the other a
somewhat pointed dome. It is noticeable that such pearls have a chalky
line around the middle, and sometimes there is a lustrous band between
two. These chalky lines are found, on peeling such a pearl, to extend
through all the interior layers. Similarly, a high button joined at
its entire circumference to the shell, if the junction is abrupt, has
an intersecting chalky line, marking the juncture of the two, between
the luster of the pearl and the shell lining. If the base of the pearl
and the shell form a curve there is no chalky line of demarcation.
This
suggests that whenever the animal is unable to envelop the thing upon
which the mantle deposits its secretions completely or is
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