a, Section of the same, magnified. b, Small portion highly magnified, showing laminæ and pillars.
The typical Stromatoporæ, or Layer-corals, consist, like Eozoon, of concentric layers, connected by numerous pillars, which are often, though not always, more definite and regular than in the Laurentian fossil. The laminæ are perforated, but more coarsely than in Eozoon, and they are often thickened with supplemental deposit which, in some of the forms, presents canals radiating from vertical tubes or bundles of tubes penetrating the mass ([Figs. 22, 23]). The mode of growth of Stromatopora must have closely resembled that of Eozoon, and the forms produced are so similar that it is often quite impossible to distinguish them by the naked eye. Like Eozoon, they form the substance of important limestones, and single masses are sometimes found as much as three feet in diameter. The Stromatoporæ extend from the Upper Cambrian to the Devonian inclusive. In the Carboniferous they are continued by smaller and more regular organisms of the genus Loftusia,[5] and this genus seems to extend without marked change up to the Eocene Tertiary. Recent students of the Stromatoporæ seem disposed to promote them from the province of Protozoa to that of the Hydroids.[6] The reasons for this seem cogent in the case of some of the forms, but in my judgment fail in others, more especially in the older forms. It may ultimately be found that the group as now held includes very different types of structure. In modern times I know of no nearer representative than the animal whose skeleton often adheres in red encrusting patches to our specimens of corals, and which is known as Polytrema. In general structure it is not very far from being a very degenerate kind of Stromatopora.
Fig. 23.—Caunopora planulata. Showing the radiating canals on a weathered surface. Devonian.—After Hall.
It is curious that in the line of succession above stated, the beautiful tubulated cell-wall of Eozoon disappears; and this structure seems, after the Laurentian, to be for ever divorced from the great laminated Protozoans. It reappears in the Carboniferous, in certain smaller organisms of the type of the Nummulites, or Money-stone Foraminifers, and is continued in this group of smaller and free animals down to the present time. In the Cretaceous and early Tertiary periods, the Foraminifera of different types have been nearly as great rock-builders as they were in the Laurentian. Some of these later rock-builders, however, have belonged to the lower or imperforate group; others to the higher or Rotaline and Nummuline groups; and, as a whole, they have been individually small, making up in numbers what they lacked in size. Probably the conditions for enabling animals of this type rapidly, and on a large scale, to collect calcareous matter, were more favourable in the Laurentian than they have ever been since.
Fig. 24.—Archæocyathus minganensis. A Primordial Protozoon.—After Billings.
a, Pores of the inner wall.