outer
shell is composed of wave-like plate extensions, superimposed one upon
the other reced-ingly from the lip to the umbo as in the others, but
without the ridges, the plates being flat and the edges more irregular.
These extensions are formed of a number of horizontal composite plates,
which penetrate the shell to the mother-of-pearl.
Not
only may they be separated into thinner horizontal plates, but they
divide vertically into prisms. Under the microscope the edge of a
composite plate appears as a number of prisms placed side by side
lengthwise across the plate edge, but showing dark, intersecting lines
through the series where they divide as plates.
These
prisms appear on the face of the plates as translucent hexagons,
separated by dark lines like a tessellated floor, and under a powerful
microscope are seen to be composed of similar smaller particles, also
joined together by a binder of tissue. The exposed parts of the
epidermis plates, forming the outer skin of the shell, are more dense
than the unexposed portions ; the hexagonal dividing lines are thick
and blurred, and the faces are almost opaque,
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