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GENESIS OF PEARLS
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called
the black shell. In others the nacre is white to the edge. The
iridescence of the white shell generally shows more play of color than
that of the black. The white shell is usually somewhat flatter and
broader than the black, and the epidermis is light yellowish-brown.
This variety is found in great abundance on the northern and western
coasts of Australia. The yellow, greenish and grayish shells (these
colors refer to the edge of the lining), are similar in every way, but
inferior, the yellow being the best of the three.
The
shell lining of a common form of the unio, or fresh water mussel
pictured at page 146, like that of the meleagrina, shows little
iridescence except at the edges outside the pallial lines, where the
nacre is comparatively thin, and at the striated surface of the scar or
bed of the adductor muscle. In quality of color and luster it is
inferior to the nacre of the sea fish, the white being more chalky in
appearance and the luster less pearly. The material of which the shell
is composed and its construction are however almost identical with that
of the saltwater mollusk. In fact all shells are made of
10 145
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