Corals. Some of the more common types are familiarly known as cup corals, honeycomb corals, and chain corals. In the cup corals the most important feature is the development of radiating vertical partitions, or septa, in the cell of the polyp. Some of the cup corals grew in hemispherical colonies (Fig. 288), while many were separate individuals (Fig. 289), building a single conical, or horn-shaped cell, which sometimes reached the extreme size of a foot in length and two or three inches in diameter.
Honeycomb corals consist of masses of small, close-set prismatic cells, each crossed by horizontal partitions, or tabulae, while the septa are rudimentary, being represented by faintly projecting ridges or rows of spines.
Chain corals are also marked by tabulae. Their cells form elliptical tubes, touching each other at the edges, and appearing in cross section like the links of a chain. They became extinct at the end of the Silurian.
The corals of the SYRINGOPORA family are similar in structure to chain corals, but the tubular columns are connected only in places.
To the echinoderms there is now added the blastoid (bud-shaped). The blastoid is stemmed and armless, and its globular “head” or “calyx,” with its five petal-like divisions, resembles a flower bud. The blastoids became more abundant in the Devonian, culminated in the Carboniferous, and disappeared at the end of the Paleozoic.
The great eurypterids—some of which were five or six feet in length—and the cephalopods were still masters of the seas. Fishes were as yet few and small; trilobites and graptolites had now passed their prime and had diminished greatly in numbers. 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.
In America the Silurian is not separated from the Devonian by any mountain-making deformation or continental uplift. The one period passed quietly into the other. Their conformable systems are so closely related, and the change in their faunas is so gradual, that geologists are not agreed as to the precise horizon which divides them.
Subdivisions and physical geography. The Devonian is represented in New York and southward by the following five series. We add the rocks of which they are chiefly composed.