structure known as _Eozooen_ is found in various localities
in the Lower Laurentian limestones of Canada, in the form of
isolated masses or spreading layers, which are composed of thin
alternating laminae, arranged more or less concentrically (fig.
22). The laminae of these masses are usually of different colours
and composition; one series being white, and composed of carbonate
of lime--whilst the laminae of the second series alternate with
the preceding, are green in colour, and are found by chemical
analysis to consist of some silicate, generally serpentine or the
closely-related "loganite." In some instances, however, all the
laminae are calcareous, the concentric arrangement still remaining
visible in consequence of the fact that the laminae are composed
alternately of lighter and darker coloured limestone.

When first discovered, the masses of _Eozooen_ were supposed to
be of a mineral nature; but their striking general resemblance
to the undoubted fossils which will be subsequently spoken of
under the name of _Stromatopora_ was recognised by Sir William
Logan, and specimens were submitted for minute examination, first
to Principal Dawson, and subsequently to Dr W. B. Carpenter.
After a careful microscopic examination, these two distinguished
observers came to the conclusion that _Eozooen_ was truly organic,
and in this opinion they were afterwards corroborated by other
high authorities (Mr W. K. Parker, Professor Rupert Jones, Mr H.
B. Brady, Professor Guembel, &c.) Stated briefly, the structure
of _Eozooen_, as exhibited by the microscope, is as follows:--

[Illustration: Fig. 23.--Diagram of a portion of _Eozooen_ cut
vertically. A, B, C, Three tiers of chambers communicating with
one another by slightly constricted apertures: _a a_, The true
shell-wall, perforated by numerous delicate tubes; _b b_. The
main calcareous skeleton ("intermediate skeleton"); c, Passage
of communication ("stolon-passage") from one tier of chambers
to another; d, Ramifying tubes in the calcareous skeleton.
(After Carpenter.)]

The concentrically-laminated mass of _Eozooen_ is composed of
numerous calcareous layers, representing the original skeleton
of the organism (fig. 23, b). These calcareous layers serve to
separate and define a series of chambers arranged in successive
tiers, one above the other (fig. 23, A, B, C); and they are
perforated not only by passages (fig. 23, c), which serve to
place successive tiers of chambers i