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
171
