Fig. 40. Archæocyathus Atlanticus—showing outer surface and longitudinal and transverse sections. (After Billings.)
Fig. 41. Structures of Archæocyathus Profundus.
(a.) Lower acervuline portion. (b.) Upper portion, with three of the radiating laminæ. (c.) Portion of lamina with pores and thickened part with canals. In figs. a and b the calcareous part is unshaded.
To understand Archæocyathus, let us imagine an inverted cone of carbonate of lime from an inch or two to a foot in length, and with its point buried in the mud at the bottom of the sea, while its open cup extends upward into the water. The lower part buried in the soil is composed of an irregular acervuline network of thick calcareous plates, enclosing chambers communicating with one another (figs. 40 and 41 A). Above this where the cup expands, its walls are composed of thin outer and inner plates, perforated with innumerable holes, and connected with each other by vertical plates, which are also perforated with round pores, establishing a communication between the radiating chambers into which they divide the thickness of the wall (figs. 38, 39, and 41 B). In such a structure the chambers in the wall of the cup and the irregular chambers of the base would be filled with gelatinous animal matter, and the pseudopods would project from the numerous pores in the inner and outer wall. In the older parts of the skeleton, the structure is further complicated by the formation of thin transverse plates, irregular in distribution, and where greater strength is required a calcareous thickening is added, which in some places shows a canal system like that of Eozoon ([fig. 41], B, C).[AJ] As compared with Eozoon, the fossils want its fine perforated wall, but have a more regular plan of growth. There are fragments in the Eozoon limestones which may have belonged to structures like these; and when we know more of the deep sea of the Primordial, we may recover true species of Eozoon from it, or may find forms intermediate between it and Archæocyathus. In the meantime I know no nearer bond of connection between Eozoon and the Primordial age than that furnished by the ancient cup Zoophytes of Labrador, though I have searched very carefully in the fossiliferous conglomerates of Cambrian age on the Lower St. Lawrence, which contain rocks of all the formations from the Laurentian upwards, often with characteristic fossils. I have also made sections of many of the fossiliferous pebbles in these conglomerates without finding any certain remains of such organisms, though the fragments of the crusts of some of the Primordial tribolites, when their tubuli are infiltrated with dark carbonaceous matter, are so like the supplemental skeleton of Eozoon, that but for their forms they might readily be mistaken for it; and associated with them are broken pieces of other porous organisms which may belong to Protozoa, though this is not yet certain.
[AJ] On the whole these curious fossils, if regarded as Foraminifera, are most nearly allied to the Orbitolites and Dactyloporæ of the Early Tertiary period, as described by Carpenter.
Of all the fossils of the Silurian rocks those which most resemble Eozoon are the Stromatoporæ, or “layer-corals,” whose resemblance to the old Laurentian fossil at once struck Sir William Logan; and these occur in the earliest great oceanic limestones which succeed the Primordial period, those of the Trenton group, in the Siluro-Cambrian. From this they extend upward as far as the Devonian, appearing everywhere in the limestones, and themselves often constituting large masses of calcareous rock. Our figure ([fig. 42]) shows a small example of one of these fossils; and when sawn asunder or broken across and weathered, they precisely resemble Eozoon in general appearance, especially when, as sometimes happens, their cell-walls have been silicified.