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THE PEARL
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on
the face, markings similar to the unexposed portions of the Venezuelan
epidermis plates only the hexagonal faces are very much smaller and
less distinct. So also the edge of the composite plate appears as
series of prisms crossing it from face to face on the plate, in sets
which show plainly, lines marking the juncture of the individual plates
or waves. Although the individual plates or waves, can only be
separated with great difficulty, together, as composite plates, they
can be flaked off from the shell very easily, and they crumble and
break into fragments under slight pressure. The component plates or
waves are very thin, and appear under the microscope as white and
translucent planes marked by outlines of the prism faces.
The
inner series of these plates as they near the nacreous lining become
harder and more compact, and incline more and more to a horizontal
position, so that at the point where they abut upon the nacre it is not
easy to distinguish them from the nacreous plates. At the thinner end
of the shell, about the edges, the plates are all of this nature. They
grow
138
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