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
enable us to decide upon its true position in the animal kingdom.
Those who wish to study this interesting subject in detail must
consult the admirable memoirs by Dr W. B. Carpenter and Principal
Dawson: it will be enough here to indicate the results which
have been arrived at. The only animals at the present day which
possess a continuous calcareous skeleton, perforated by pores
and penetrated by canals, are certain organisms belonging to
the group of the _Foraminifera_.
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