[Illustration:  Fig. 22.—­Fragment of Eozooen, of the natural size, showing alternate laminae of loganite and dolomite. (After Dawson.)]

The 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 in 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