=The continents and oceans which arose at the close of the Paleozoic era.=—At the close of the second great era in the recorded history of the earth, the now somewhat enlarged continents were profoundly altered during a series of convulsive movements within the surface shell of the lithosphere. When these convulsions were over, there was a new disposition of land and sea, but one quite different from the present arrangement. Instead of being extended in north-south belts, as they are at present, the continents stretched out in broad east-west zones, one in the northern and the other in the southern hemisphere. To the broad southern continent of which so little now remains, the name “Gondwana Land” has been given, and to the sea which divided the northern from the southern continent the name “Ocean of Tethys.” The northern continent stretched across the site of the present Atlantic Ocean as the “North Atlantis”, its northern shore to the westward being somewhat farther south than the present northern coast of North America, since life forms migrated in the northern ocean from the site of Behring Sea to that of the present North Atlantic. This arrangement of land and water during the middle period of the earth’s recorded history, when considered with reference both to its earlier and to its later evolution, may perhaps be best accounted for by the assumption that the lithosphere was then shaped like Fig. 5 (middle). In this figure two truncated tetrahedrons are joined in a common plane of contact which may be described as the twin plane. This medial depression upon the lithosphere was occupied by the intercontinental sea, the Ocean of Tethys. Near the close of this second great era of the earth’s continental history, crustal convulsions, which were perhaps the most remarkable in the entire record, resulted in the almost complete disappearance of the southern continent and a concentration of the land within the northern hemisphere as a somewhat interrupted belt surrounding a central polar ocean (Figs. 3 and 5). Upon the assumption of twin tetrahedrons in the intermediate era of continental evolution, both the Ocean of Tethys of that time and its present remnants, the Caribbean and Mediterranean seas, are accounted for. The V-shaped continent extensions and the A-shaped oceans of the southern hemisphere (Fig. 2) may likewise be considered as relics of the now largely submerged tetrahedron of the southern hemisphere, since this had its apex to the northward (Fig. 6). [Illustration: FIG. 6.—Diagrams for comparison of shore lines upon tetrahedrons which have an angle, the first at the south and the second at the north.]