The genus Stromatopora of Goldfuss was defined by him as consisting of laminæ of a solid and porous character, alternating and contiguous, and constituting a hemispherical or sub-globose mass. In this definition, the porous strata are really those of the fossil, the alternating solid strata being the stony filling of the chambers; and the descriptions of subsequent authors have varied according as, from the state of preservation of the specimens or other circumstances, the original laminæ or the filling of the spaces attracted their attention. In the former case the fossil could be described as consisting of laminæ made up of interlaced fibrils of calcite, radiating from vertical pillars which connect the laminæ. In the latter case, the laminæ, appear as solid plates, separated by very narrow spaces, and perforated with round vertical holes representing the connecting pillars. These Stromatoporæ range from the Lower Silurian to the Devonian, inclusive, and many species have been described; but their limits are not very definite, though there are undoubtedly remarkable differences in the distances of the laminæ and in their texture, and in the smooth or mammillated character of the masses. Hall's genus Stromatocerium belongs to these forms, and D'Orbigny's genus Sparsispongia refers to mammillated species, sometimes with apparent oscula. Phillip's genus Caunopora was formed to receive specimens with concentric cellular layers traversed by "long vermiform cylindrical canals;" while Winchell's genus Cœnostroma includes species with these vermiform canals arranged in a radiate manner, diverging from little eminences in the concentric laminæ. The distinction between these last genera does not seem to be very clear, and may depend on the state of preservation of the specimens. A more important distinction appears to exist between those that have a single vertical canal from which the subordinate canals diverge, and those that have groups of such canals. Some species of the Cœnostroma group have very dense calcareous laminæ traversed by the canals; but it does not seem that any distinction has yet been made between the proper wall and the intermediate skeleton; and most observers have been prevented from attending to such structures by the prevailing idea that these fossils are either corals or sponges, while the state of preservation of the more delicate tissues is often very imperfect. (B.) Localities of Eozoon, or of Limestones supposed to contain it.