66 THE BOOK OF THE PEARL
North
America, pearls are found in the pearl-oyster of the Gulf of
California, the abalone of the Pacific coast, the queen conch of the
Gulf of Mexico, and in the Unios of most of the rivers, especially
those of the Mississippi Valley.
Since
pearly concretions partake of the characteristics of the shell within
which they are formed, it follows that practically all species of
mollusks whose shells have a well-developed nacreous lining yield
pearls to a greater or less extent. But the number of these species is
relatively small. They belong chiefly to the Margaritiferœ, or pearl-oyster family of the sea, and to the Unionidœ, or
family of fresh-water mussels. Pearls occur also in some univalves, but
not so abundantly as in bivalves of the families mentioned. Broadly
stated, we may hope to find pearls within any mollusk whose shell
possesses a nacreous surface ; and it is useless to search for them in
shells whose interior is dull and opaque, such as the edible oyster for
instance.
The great bulk of the pearls on the market, and likewise those of the highest quality, are from the Margaritiferœ, which
are widely distributed about tropical waters. Although these mollusks
are spoken of as pearl-oysters, they are not related in any way to the
edible oysters (Ostrea) of America and Europe.1 The
flesh is fat and glutinous, and so rank in flavor as to be almost unfit
for food, although eaten at times by the poorer fishermen in lieu of
better fare. The origin of the name is doubtless due to the fact that
in the somewhat circular form of the shell they resemble oysters rather
than the elongated mussels of Europe, to which they are more nearly
related in anatomy. Also in that—like their namesakes—they are
monomyarian, having only one adductor muscle.
The
two valves or sides of the pearl-oyster shell are nearly similar in
shape and almost equal in size ; whereas in the edible oysters one
valve is thin and somewhat flat, while the other is thicker, larger,
and highly convex. In the latter, also, the hinge, or umbo, is an
angular beak ; but in the pearl-oysters the umbo is prolonged by
so-called ears or wings into a straight line the length of which is
nearly equal to the breadth of the shell.
The
byssus, or bunch of fibers, by which pearl-oysters attach themselves
to the bottom indicates their relationship to the mussels. The
possession of a small foot and somewhat extended migratory powers
