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BRYOZOANS. No fossils are more common in the limestones of the time than the small branching stems and lacelike mats of the bryozoans, the skeletons of colonies of a minute animal allied in structure to the brachiopod. BRACHIOPODS. These multiplied greatly, and in places their shells formed thick beds of coquina. They still greatly surpassed the mollusks in numbers.
The Globigerinae, Cyatholiths, Coccospheres, Discoliths in the one are absolutely identical with those in the other; there are identical, or closely analogous, species of Sponges, Echinoderms, and Brachiopods.
Thus the brachiopods, a type of shellfish, have made no progress since the Paleozoic, and some of their earliest known genera are represented by living forms hardly to be distinguished from their ancient ancestors. The lowest and earliest branches of the tree of life have risen to no higher levels since they reached their climax of development long ago.
They differ from the Brachiopods chiefly in the higher character of their breathing-apparatus; for they have free gills, instead of the net-work of vessels on the lining skin which serves as the organ of respiration in the Brachiopods.
One of these sketches shows us such a creature hungrily inspecting a pool where Crinoids, with their long stems, large, closely-coiled Chambered Shells, and Brachiopods, the Oysters and Clams of those days, offer him a tempting repast.
Below this Devonian limestone, its crystals sparkling in the sunshine, with its coral fossils, its fragments of crinoids, and its broken shells of brachiopods, down through the Devonian, the Silurian, the Ordovician, and the Cambrian rocks, down to the original crust formed when first the earth began to cool, if any there be remaining; all these miles of rocks are inorganic, built up of crystals.
Scorpions are found in this period both in Europe and in America. The limestone-making seas of the Silurian swarmed with corals, crinoids, and brachiopods. With the end of the Silurian period the AGE OF INVERTEBRATES comes to a close, giving place to the Devonian, the AGE OF FISHES.
They would likewise be fatal, if the above Foraminifera, for instance, could be proved to have first come into existence during the Laurentian epoch, or the above Brachiopods during the Cambrian formation; for in this case, there would not have been time sufficient for the development of these organisms up to the standard which they had then reached.
At an ancient epoch the highest and lowest molluscoidal animals, namely, cephalopods and brachiopods, swarmed in numbers; at the present time both groups are greatly reduced, while others, intermediate in organisation, have largely increased; consequently some naturalists maintain that molluscs were formerly more highly developed than at present; but a stronger case can be made out on the opposite side, by considering the vast reduction of brachiopods, and the fact that our existing cephalopods, though few in number, are more highly organised than their ancient representatives.
In the great sub-tropical sea which once covered this spot, corals lived and flourished as they do now in similar seas. Myriads of brachiopods lived, moved, and had their being. Gigantic fish sported in the waters. Meanwhile older rocks were being denuded and disintegrated. Millions of tons of sediment were brought by the rivers and streams to the shores of the Devonian sea.
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