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STRUCTURE AND FORMS
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57
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the
pearl, its position being recognized only by the excrescence on one
side, and, with continued increase in size, even this may be almost
overcome.
Sometimes
double, triple, or multiple pearls are formed; each of these may have a
separate nucleus and grow independently for a time until they adjoin
each other ; continuing to grow, they become so united as to form a
connected mass. The "Southern Cross" is a remarkable example of this.
It appears to consist of seven nearly spherical pearls attached to one
another in a straight line, and one projecting from each side of the
second in the row, thus forming a Roman cross.1
A
few years ago, near Sharks Bay, on the coast of western Australia, a
cluster was found containing about 150 pearls closely compacted. This
cluster measured about one and a half inches in length, three quarters
of an inch in breadth, and half an inch in thickness.
When
a growing pearl is very near to the nacreous lining of the shell, the
pressure between the two hard substances results in a rupture of the
pearl-forming sac and the epithelial layer of the shell, and the pearl
comes in actual contact with the nacre. The pearl gradually becomes
attached to the shell, and the under portion is prevented from growing
further; the upper or exposed surface receives other layers, resulting
in the formation of a bouton. As the shell around the pearl
continues to grow, it gradually closes about, and almost wholly
conceals the pearl. Since it is constantly wasting away on the exterior
surface as it grows on the interior, it follows that in time the shell
passes the pearl quite through to the outside, where it rapidly decays.
Thus the oyster virtually forces the annoying intruder directly through
the wall of its house instead of by way of the open door, and magically
closes the breach with its marvelous masonry.
These
embedded pearls are generally faulty and of diminished luster, but in
the aggregate, large quantities of imperfect ones, and especially half
and quarter pearls, are secured in this manner. Sometimes—particularly
in the Australian fisheries—large pearls are thus found, weighing
twenty, forty, sixty, and even eighty grains; and when the faulty
outside layers of nacre are removed, a subjacent surface of fine
luster may possibly be revealed. In bivalves, these adherent pearls
are commonly in the deep or lower valve, except in those unusual cases
where the mollusks have been lying in a reverse position. At the
fisheries, the surfaces of the shells are carefully inspected for
evidence of pearly nodules, and these are broken open in search for
encysted objects. Cutters of mother-of-pearl occasionally find
embedded pearls of this kind which have escaped the vigilant eyes of
the fishermen.
*See p. 465.
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