n communication, but also by
a system of delicate branching canals (fig. 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. In some cases
the chambers are simply filled with crystalline carbonate of
lime. When the originally porous fossil has been permeated by
a silicate, it is possible to dissolve away the whole of the
calcareous skeleton by means of acids, leaving an accurate and
beautiful cast of the chambers and the tubes connected with them
in the insoluble silicate.

[Illustration: Fig. 24.--Portion of one of the calcareous layers
of _Eozooen_, magnified 100 diameters. a a, The proper wall
("Nummuline layer") of one of the chambers, showing the fine
vertical tubuli with which it is penetrated, and which are slightly
bent along the line a' a'. c c, The intermediate skeleton,
with numerous branched canals. The oblique lines are the cleavage
planes of the carbonate of lime, extending across both the
intermediate skeleton and the proper wall. (After Carpenter.)]

The above are the actual appearances presented by _Eozooen_ when
examined microscopically, and it remains to see how far they
en