By the dawn of this period of relative quiet on the earth's surface, life is confined to the various inland seas and the oceanic shore line; as yet no form of land organism has evolved. Primitive marine animals are well established and are prepared for the next evolutionary development. Amoeba are typical survivors of this initial stage of animal life, having made their appearance toward the close of the preceding transition period.

400,000,000 years ago marine life, both vegetable and animal, is fairly well distributed over the whole world. The world climate grows slightly warmer and becomes more equable. There is a general inundation of the seashores of the various continents, particularly of North and South America. New oceans appear, and the older bodies of water are greatly enlarged.

Vegetation now for the first time crawls out upon the land and soon makes considerable progress in adaptation to a nonmarine habitat.

Suddenly and without gradation ancestry the first multicellular animals make their appearance. The trilobites have evolved, and for ages they dominate the seas. From the standpoint of marine life this is the trilobite age.

In the later portion of this time segment much of North America and Europe emerged from the sea. The crust of the earth was temporarily stabilized; mountains, or rather high elevations of land, rose along the Atlantic and Pacific coasts, over the West Indies, and in southern Europe. The entire Caribbean region was highly elevated.

390,000,000 years ago the land was still elevated. Over parts of eastern and western America and western Europe may be found the stone strata laid down during these times, and these are the oldest rocks which contain trilobite fossils. There were many long fingerlike gulfs projecting into the land masses in which were deposited these fossil-bearing rocks.

Within a few million years the Pacific Ocean began to invade the American continents. The sinking of the land was principally due to crustal adjustment, although the lateral land spread, or continental creep, was also a factor.

380,000,000 years ago Asia was subsiding, and all other continents were experiencing a short-lived emergence. But as this epoch progressed, the newly appearing Atlantic Ocean made extensive inroads on all adjacent coast lines. The northern Atlantic or Arctic seas were then connected with the southern Gulf waters. When this southern sea entered the Appalachian trough, its waves broke upon the east against mountains as high as the Alps, but in general the continents were uninteresting lowlands, utterly devoid of scenic beauty.

The sedimentary deposits of these ages are of four sorts:

1. Conglomerates—matter deposited near the shore lines.

2. Sandstones—deposits made in shallow water but where the waves were sufficient to prevent mud settling.

3. Shales—deposits made in the deeper and more quiet water.

4. Limestone—including the deposits of trilobite shells in deep water.

The trilobite fossils of these times present certain basic uniformities coupled with certain well-marked variations. The early animals developing from the