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GENESIS OF PEARLS
It is of the black-edge variety, contains a large quantity of fine quality mother-of-pearl, and has a beautiful small pearl attached to the lining near the center of the shell. Though large, it is not full grown. It is probably twelve to fourteen years old and would continue to lay on mother-of-pearl and so grow thicker and heavier until sixteen to eighteen years of age, when the oyster would reach maturity. The Australian white shell at page 129 is a young shell—that is, it has not attained the full thick­ness and weight of a mature shell. The shells at pages 131 and 161 are from the coast of Venezuela; they measure i\ by 2\ inches and weigh seven pennyweights each.
The common form of the pearl-bearing fresh­water mussel unio (nigger-head) is illustrated at page 146. This shell measures 3! by i\ inches and weighs 3! ounces. It is from the Middle West of the United States. In con­struction it resembles the meleagrina, the epidermis being dark, though not as rough as that of the oyster, and the lining white, show­ing slight iridescence around the lip-edge and to a greater degree on the adductor muscle
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