literally
"rainbow-hued"—that, is presented by many of these fossils from
Jurassic and cretaceous deposits. Among the gasteropods, the pearly
groups are the Turbos and Haliotes, in both of which, but especially in
the latter, there is a frequent occurrence of green iridescence. Shells
of both these families are " cleaned " with acid for use as ornaments,
and the exquisite green Haliotis material is extensively used in the
arts, as described further on.
The
pearls of commerce, however, are almost wholly obtained from bivalve
(lamellibranch) shells, of which the following families have a
nacreous lining: Aviculidse, Mytilidse, and Unionidae, the latter being
wholly fresh-water shells, also known as the Naiads. A few genera of
other families are also brilliantly pearly, but need not be discussed.
The true pearl-oyster (Meleagrina) of the Pacific and Indian Oceans
belongs to the first of these groups, and has from time immemorial
yielded the bulk of commercial pearls, while its large and thick shell
furnishes the mother-of-pearl for countless ornamental purposes. (The
Naiades are of particular interest in this country, as it is in North
America that this group is most abundant.) Several hundred species of
Unio, Anodon, etc., have been found in our great rivers and lakes, and
the Mississippi basin teems with them; and for the most part the forms
are quite distinct from those of the Atlantic watershed and of the Old
World. The Unios, while all iridescent, vary greatly in tint,
exhibiting all the delicate shades of pink, brown, purple, etc., as
well as white. The rivers of Europe, of Mesopotamia, and of China also
yield large numbers of Unios.
The
peculiar artificial devices for pearl production employed by the
Chinese with Dipsas plicatus are described hereafter in this chapter,
as well as similar experiments upon Unios in Germany. Other genera
(Hyria and Castalia) represent the family in the Amazon basin of South
America.
The
same causes and operations that result in the production of pearls or
free nacreous concretions in the soft animal substance of the pearl
oysters or mother-of-pearl shells also produce in a modified way the
tuberculose or knoblike protuberances and irregularities of surface
that are frequently seen on
