=The western United States.= So little is found of the rocks of the system west of the Missouri River that it is quite probable that the western part of the United States had for the most part emerged from the sea at the close of the Ordovician and remained land during the Silurian. At the same time the western land was perhaps connected with the eastern land of Appalachia across Arkansas and Mississippi; for toward the south the Silurian sediments indicate an approach to shore. _Life of the Silurian_ In this brief sketch it is quite impossible to relate the many changes of species and genera during the Silurian. =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. [Illustration: Fig. 288. A Compound Cup Coral] [Illustration: Fig. 289. A Simple Cup Coral] [Illustration: Fig. 290. Honeycomb Corals] [Illustration: Fig. 291. A Chain Coral] [Illustration: Fig. 292. A Syringopora Coral] _Honeycomb corals_ consist of masses of small, close-set prismatic cells, each crossed by horizontal partitions, or _tabulæ_, while the septa are rudimentary, being represented by faintly projecting ridges or rows of spines. _Chain corals_ are also marked by tabulæ. 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. [Illustration: Fig. 293. A Blastoid: A, side view, showing portion of the stem; B, summit of calyx (species Carboniferous)] [Illustration: Fig. 294. A Silurian Scorpion] 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.