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THE PEARL
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points
only. Others are faulty in shape, or if spherical, deeply pitted.
Really fine pieces are usually small or button, and when large, are
baroques. Some of the latter are magnificent. Weighing fifty to over
one hundred grains, with skins of extraordinary luster and iridescence;
white, or of a beautiful pink tint, these strawberry or rose pearls,
as they are called, frequently excel, by every standard of beauty, the
imperfect spheres which command a greater price in the market because
they are round.
The
most common variety of unio in American rivers, especially in the
Mississippi river, is that known as the nigger-head (Quadrula ebena).
It is also the principal species used for button-making.
Similar
is the warty-back (Quadrula pustu-losa) so called because the shell has
a number of warts or excrescences on the outside of the valves. The ''
bull-head'' (Pleurobena Aesopus) is found in abundance with the
nigger-head. It has a blackish-brown exterior, presenting several
radiating ridges, and a white lining. The two latter are inferior as
material for buttons as the shells are brittle. The mucket
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