Lower
Silurian period, if not earlier, and they have not only held their own,
but have gradually increased in number and variety of type, up to the
present day. The family of Aviculidce flourished in the Carboniferous period, and beds containing immense numbers of Avicula Contorta occur
in the Triassic or Rhoetic series, in the Austrian Alps. The more
interesting Gasteropods and Cephalopods also have their histories
clearly marked out from a very early date. Bivalves have undoubtedly,
been most successful in the struggle for existence, and this power may
be partly attributed to their ability of closing their shells when
attacked, and presenting an impenetrable front of " masterly
inactivity."
It
is needless to enumerate the articles for the manufacture of which
Pearl shell is sought after. For buttons and studs, for knife-handles,
card-cases, and for ornamental work generally, Mother-of-Pearl has no
rival. Its adoption is of no modern date, articles of this substance
having been discovered in the excavations at Nineveh and Babylon.
One
important application of Mother-of-Pearl is that of hafting cutlery,
especially fruit knives and pocket knives. The two flat plates of
shell, which are rivetted to the central part of the handle, are
technically termed "scales," and these require to be tediously ground
down and polished by hand.
