23, d). Moreover,
the central and principal portion of each calcareous layer, with
the ramified canal-system just spoken of, is bounded both above
and below by a thin lamina which has a structure of its own, and
which may be regarded as the proper shell-wall (fig. 23, a a).
This proper wall forms the actual lining of the chambers, as well
as the outer surface of the whole mass; and it is perforated with
numerous fine vertical tubes (fig. 24, a a), opening into the
chambers and on to the surface by corresponding fine pores. From
the resemblance of this tubulated layer to similar structures
in the shell of the Nummulite, it is often spoken of as the
"Nummuline layer." The chambers are sometimes piled up one above
the other in an irregular manner; but they are more commonly
arranged in regular tiers, the separate chambers being marked
off from one another by projections of the wall in the form of
partitions, which are so far imperfect as to allow of a free
communication between contiguous chambers. In the original condition
of the organism, all these chambers, of course, must have been
filled with living-matter; but they are found in the present
state of the fossil to be generally filled with some silicate,
such as serpentine, which not only fills the actual chambers,
but has also penetrated the minute tubes of the proper wall and
the branching canals of the intermediate skeleton.
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