In the Siluro-Cambrian age two other forms of gigantic Foraminiferal Protozoans were introduced, widely different from Eozoon, and destined apparently not to survive the period in which they appeared. These were Archæocyathus, the ancient Cup-corals, and Receptaculites, which may perhaps be called the Sack-corals. Both are quite remote from Eozoon in structure, wanting its complexity in the matter of minute tubules, and having greater regularity and complication on the large scale. Archæocyathus had the form of a hollow inverted cone with double perforated walls, connected by radiating irregular plates, also perforated ([Fig. 24]). It has been regarded as a sponge, and some species are certainly accompanied with spicules; but these I have ascertained to be merely accidental, and will be referred to in the next chapter. The true structure of Archæocyathus consists of radiating calcareous plates enclosing chambers connected by pores. Archæocyathus came in with the Later Cambrian, and seems to have died out in the Siluro-Cambrian. The only more modern things which at all resemble it are the Foraminifera called Dactylopora, which belong to the Tertiary period.
Fig. 25.—Receptaculites. Restored.—After Billings.
a, Aperture. b, Inner wall. c, Outer wall. n, Nucleus, or primary chamber. v, Internal cavity.
Receptaculites is a still more complex organism. It has a sack-like form, often attaining a large size, and the double walls are composed of square or rhombic plates, connected with each other by hollow tubes from which proceed canals perforating the plates ([Fig. 25]). This curious structure is confined to the Siluro-Cambrian, and is so dissimilar from modern forms that its affinities have been subject to grave doubts.
Fig. 26.—Section of Loftusia Persica. An Eocene Foraminifer. Magnified five diameters.—After Carpenter and Brady.
We thus have presented to us the remarkable fact that in the Palæozoic age we have no precise representative of Eozoon, but instead three divergent types, differing from it and from each other, all apparently specialised to particular uses, all temporary in their duration; while in later times nature seems to have returned nearer to the type of Eozoon, though on a smaller scale, and separating some characters conjoined in it. Some portion of this curious result may be due to our ignorance; and it would be interesting to know, what we may know some day, how this type of life was represented in the long interval between the Huronian and the Upper Cambrian, when perhaps there may have been forms that would at least enable us to connect Eozoon and Stromatopora.