The probable distribution of land and sea in eastern North America and western Europe is shown in Figure 287. The fauna of the interior region and of eastern Canada are closely allied with that of western Europe, and several species are identical. We can hardly account for this except by a shallow-water connection between the two ancient epicontinental seas. It was perhaps along the coastal shelves of a northern land connecting America and Europe by way of Greenland and Iceland that the migration took place, so that the same species came to live in Iowa and in Sweden. 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. 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.