SHELL.
559
striation is seen to be a thickening of the cell- thickness of the shell is made up of the inter¬
wall in those situations ; which will of course nal or nacreous layer ; but a uniform stratum
Fig. 411.
Calcareous prisms of the shell of Pinna ; from Chalk.
produce a corresponding series of indentations
upon the contained prisms. This thickening
seems best accounted for by supposing (as
first suggested by Prof. Owen) that each long
prismatic cell is made up by the coalescence of
a pile of flat epidermic cells, the transverse
striation marking their lines of junction ; and
this view corresponds well with the fact that
the shell-membrane not unfrequently shows a
tendency to split into thin laminae along the
lines of striation, as shown in the iower part
offig. 410; whilst we occasionally meet with an
excessively thin natural lamina, composed of
flat pavement-like cells resembling those of
the epithelium of serous membrane, lying
between the thicker prismatic layers, with one
of which it would have probably coalesced
but for some accidental cause which pre¬
served its distinctness. That the entire length
of the prism is not formed at once, but that it
is progressively lengthened and consolidated
at its lower extremity, would appear also from
the fact that where the shell presents a deep
colour (as in Pinna nigrina) this colour is
usually disposed in distinct strata, the outer
portion of each layer being the part most
deeply tinged, whilst the inner extremities of
the prisms are almost colourless.
The prismatic arrangement of the carbonate
of lime in the shells of Pinna and its allies
has been long familiar to concholdgists ; but
it has been usually regarded as the result of
crystallisation. It is now, however! perfectly
evident that the calcareous prisms a'e nothing
else than casts of the interior of the prismatic
cells; the form of which, however irregular,
they constantly present ; whilst the markings
of the membrane are faithfully transferred to
the surface of the prism. Further, the prisms
in a thick layer of shell frequentlyfpresent a
decided curvature, which would not be the
case if their form were due to crystallisa¬
tion. Not unfrequently, moreover, they are
altogether destitute of angular boundaries ;
the large quantity of animal matter disposed
between the contiguous cells giving them a
rounded contour, as seen in fig. 412, and thus
causing the calcareous casts of thejr interior
to be cylindrical rather than prismatic.
It is only in a few families of Bivalves, how¬
ever, that the cellular structure is seen in this
very distinct form, or that it makes up a large
part of the substance of the shell ; and these
families are for the most part nearly allied to
Pinna. In all the genera of the Margaritaceœ,
we find the external layer of the shell formed
upon this plan, and of considerable thickness ;
the internal layer being nacreous. In the
Unionidee, on the contrary, nearly the whole
Fig. 412.
Lamina of outer layer of shell of Ostrea edulis,
showing its cellular structure, with a large amount
of intercellular substance. Magnified 250 diameters.
of prismatic cellular substance is always found
between the nacre and the periostracum. In
the Ostraceæ the greater part of the shell is
composed of a sub-nacreous substance, the
successively-formed laminas of which have very
little adhesion to each other ; but every one
of these laminae is bordered at its free edge by
a layer of the prismatic cellular substance,
distinguished by its brownish-yellow colour:
this structure presents itself again in the family
Pandoridœ, which belongs to quite a different
section of the class ; and it is curious to ob¬
serve that the marked difference in the struc¬
ture of the shells of Pandora and Lyonsia from
that of the Anatinidæ and other neighbouring
families, harmonises completely with the pecu¬
liar combination of characters presented by-
the animals of these two genera.* In all the
foregoing cases, a distinct cellulo-membranous
residuum is left after the décalcification of the
prismatic substance by dilute acid ; and this is
most tenacious and'substantial where, as in
the Margaritaceœ, there is no proper perios¬
tracum, — as if the horny matter which would
have otherwise gone to form this investment
had been diffused as an intercellular substance
between the proper cell-walls.
In many other instances, a cellular arrange¬
ment is perfectly evident in sections of the
shell ; and yet no corresponding structure
can be distinctly seen in the delicate membrane
left after décalcification. In all such cases
the animal basis bears but a very small propor¬
tion to the calcareous deposit, and the shell is
usually extremely hard. A very characteristic
example of this is presented by the outer layers
of the shells of the genus Thracia and other
Anatinidæ. But there are numerous other
cases, in which no traces of cellular structure
can be detected in the fully-formed shell, and
in which we can only be guided by analogy in
* See Forbes and Hanley’s British Mollusca,
vol. i. pp. 207, 213.