bottom
was thoroughly cleaned, this fact, if the above explanation be correct,
would point to a far more rapid growth of oyster shell than pearlers
have hitherto thought possible.
It
is often maintained in Australia, that as certain spots are left at the
end of one season, rich in shell, and six months afterwards the
identical spots are found bare, the oysters must possess the power of
migrating at will. There is, however, no evidence of value to lead to
such a conclusion, and the inference from the disappearance of shells
is, that some change has taken place at the bottom of the sea, the
shells having been probably buried by sand or mud, which doubtless is
continually shifting, especially where the tides are strong and the sea
shallow.
The
geographical distribution of the true Mother-of-Pearl shell is confined
to the Pacific and Indian Oceans and their connecting seas. Cape Horn,
and the Cape of Good Hope stretching away down into cold latitudes,
appear effectually to have prevented their successful migration in the
present age, by any favouring current to the Atlantic.
The
ancient history of the Mollusca, as told by Geology, is well worth
studying. The Lamelli-branchiata or Conchifera, existed as far back as
the
