The structure known as _Eozoön_ 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 laminæ, arranged more or less concentrically (fig. 22). The laminæ of these masses are usually of different colours and composition; one series being white, and composed of carbonate of lime--whilst the laminæ 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 laminæ are calcareous, the concentric arrangement still remaining visible in consequence of the fact that the laminæ are composed alternately of lighter and darker coloured limestone. When first discovered, the masses of _Eozoön_ 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 _Eozoön_ 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 Gümbel, &c.) Stated briefly, the structure of _Eozoön_, as exhibited by the microscope, is as follows:-- [Illustration: Fig. 23.--Diagram of a portion of _Eozoön_ 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.)]