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.