more
friable and chalky as they incline to the perpendicular, where the
series are more numerous and are situated at the thicker part of the
shell about the umbo.
Adjoining
the inner edges of the middle shell plates is the nacreous lining. In
this the calcium carbonate takes the same form as the mineral aragonite
and is identical with it. As a mass however, the specific gravity is
somewhat less, owing to the inclusion of organic matter with the
mineral in the shell. This material is harder, finer, more compact, and
contains less organic matter than that of which the middle and outer
shell is composed.
The
lining is constructed of thin waves of transparent calcium carbonate
set in animal tissue of great tenuity. This is the mother-of-pearl, and
the gem differs from it only in its more or less rounded and
independent formation. The plates of which the lining is composed lie
almost parallel to the plates of the epidermis. They are bent a little
toward the interior at the inner surface of the shell, but the general
sectional structure of a shell, cutting from the umbo to the lip, is
fairly represented by that
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