parallel
to the shell surface. The last two strata consist chiefly of carbonate
of lime. These formations were illustrated by transverse cuttings and
microscopic sections. (See Fig. 8.) When a wound had been received by the
animal in any soft part, the tissues became moistened with a lime-like
material and especially with the nacre-substance. This often happens in
the muscles which serve to close the shell, and the irregular
concretions thus formed are called "sand pearls." When the growth of
the pearl is abnormally strong, the pressure which it exerts on the
outer wall of this tissue-pocket becomes so powerful that the pocket
is absorbed on the side toward the shell, bringing the hard pearl
directly against it. It then becomes impossible for the pearl to grow
any more at the point of contact, for there is no tissue to secrete the
lime substance ; but it grows on the rest of the surface, and the
thickening layers, as they are formed, pass directly into the nacre
layers on the inside of the shell, and thicken the shell itself.
Through these over-layers, the pearl is connected with the shell as
though by different layers of covering cloths. At first it clings to
the shell at one point only, afterward enlarging the area of its
adhesion. In this manner twin or united pearls are formed.
The
most important marine pearl-fishery on the American continent is that
of Lower California, the central point being at La Paz. Here the true
pearl oysters, Meleagrina or Margarito-phora, are found, on the eastern
shores of the Gulf of California, from Cape St. Lucas to the mouth of
the Colorado River, taking in about 1,500 miles of coast, including thé
gulf islands. They are also found from La Barra de Ocoz, which is the
boundary line between the republics of Guatemala and Mexico, to
Mazatlan, a distance of 2,000 miles, making for the pearl fisheries a
total extent of 3,500 miles.
These
fisheries have recently been confirmed to the Pearl Shell Company of
San Francisco, by special franchise from the Mexican Government. The
beds were first discovered some three centuries ago by Hernando Cortez
when he crossed
