| Fig. 44. Caunopora planulata, Hall—Devonian; showing the radiating canals on a weathered surface. (After Hall.) | Fig. 45. Cœnostroma—Guelph Limestone, Upper Silurian, from a specimen collected by Mr. Weston, showing the canals. (a.) Surface with canals, natural size. (b.) Vertical section, natural size. (c.) The same magnified, showing canals and laminæ. |
There are probably many species of these curious fossils, but their discrimination is difficult, and their nomenclature confused, so that it would not be profitable to engage the attention of the reader with it except in a note. Their state of preservation, however, is so highly illustrative of that of Eozoon that a word as to this will not be out of place. They are sometimes preserved merely by infiltration with calcite or dolomite, and in this case it is most difficult to make out their minute structures. Often they appear merely as concentrically laminated masses which, but for their mode of occurrence, might be regarded as mere concretions. In other cases the cell-walls and pillars are perfectly silicified, and then they form beautiful microscopic objects, especially when decalcified with an acid. In still other cases, they are preserved like Eozoon, the walls being calcareous and the chambers filled with silica. In this state when weathered or decalcified they are remarkably like Eozoon, but I have not met with any having their minute pores and tubes so well preserved as in some of the Laurentian fossils. In many of them, however, the growth and overlapping of the successive amœba-like coats of sarcode can be beautifully seen, exactly as on the surface of a decalcified piece of Eozoon. Those in my collection which most nearly resemble the Laurentian specimens are from the older part of the Lower Silurian series; but unfortunately their minute structures are not well preserved.
In the Silurian and Devonian ages, these Stromatoporæ evidently carried out the same function as the Eozoon in the Laurentian. Winchell tells us that in Michigan and Ohio single specimens can be found several feet in diameter, and that they constitute the mass of considerable beds of limestone. I have myself seen in Canada specimens a foot in diameter, with a great number of laminæ. Lindberg[AK] has given a most vivid account of their occurrence in the Isle of Gothland. He says that they form beds of large irregular discs and balls, attaining a thickness of five Swedish feet, and traceable for miles along the coast, and the individual balls are sometimes a yard in diameter. In some of them the structure is beautifully preserved. In others, or in parts of them, it is reduced to a mass of crystalline limestone. This species is of the Cœnostroma type, and is regarded by Lindberg as a coral, though he admits its low type and resemblance to Protozoa. Its continuous calcareous skeleton he rightly regards as fatal to its claim to be a true sponge. Such a fossil, differing as it does in minute points of structure from Eozoon, is nevertheless probably allied to it in no very distant way, and a successor to its limestone-making function. Those which most nearly approach to Foraminifera are those with thick and solid calcareous laminæ, and with a radiating canal system; and one of the most Eozoon-like I have seen, is a specimen of the undescribed species already mentioned from the Guelph (Upper Silurian) limestone of Ontario, collected by Mr. Weston, and now in the Museum of the Geological Survey. I have attempted to represent its structures in [fig. 44].
[AK] Transactions of Swedish Academy, 1870.
In the rocks extending from the Lower Silurian and perhaps from the Upper Cambrian to the Devonian inclusive, the type and function of Eozoon are continued by the Stromatoporæ, and in the earlier part of this time these are accompanied by the Archæocyathids, and by another curious form, more nearly allied to the latter than to Eozoon, the Receptaculites. These curious and beautiful fossils, which sometimes are a foot in diameter, consist, like Archæocyathus, of an outer and inner coat enclosing a cavity; but these coats are composed of square plates with pores at the corners, and they are connected by hollow pillars passing in a regular manner from the outer to the inner coat. They have been regarded by Salter as Foraminifers, while Billings considers their nearest analogues to be the seed-like germs of some modern silicious sponges. On the whole, if not Foraminifera, they must have been organisms intermediate between these and sponges, and they certainly constitute one of the most beautiful and complex types of the ancient Protozoa, showing the wonderful perfection to which these creatures attained at a very early period. ([Figs. 46, 47, 48.])
Fig. 46. Receptaculites, restored. (After Billings.)
(a.) Aperture. (b.) Inner wall. (c.) Outer wall. (n.) Nucleus, or primary chamber. (v.) Internal cavity.