Page:Popular Science Monthly Volume 80.djvu/402

398 of widely separated faunas. A large proportion of the Karnic species of India is identical with the Mediterranean fauna, and in California about one third are common forms in the Alps and Sicily.

Immediatelly above the beds with Tropites subbullatus in India, the Alps, and in California occur massive limestones full of coral reefs, with some few identical species in all three regions. This same group of ancient reef-builders has also been found in Alaska, in the same stratigraphic position. This shows that the wide distribution of the Upper Triassic species was made possible by the nearly uniform distribution of warm water over a great part of the globe.

In the latter part of the Upper Triassic widespread physiographic disturbances had again divided the great zoologic regions. The Atlantic-Pacific connection had disappeared, but there was still free communication between the Alpine province and the Orient, for Mediterranean species reached as far as India and Timor. Around the North Pacific, with the Arctic sea as its center of dispersion, a different fauna, that of the bivalve, Pseudomonotis ochotica, was distributed. This extended from northern Siberia southwestward to the Crimea, southeastward to the Great Basin Sea, and southward along the shores of Asia through Japan to New Zealand.

In the Lower Jurassic we find a recurrence of the same conditions that prevailed in the Karnic epoch, free communication through the Central American and Asia Minor portals, and between the Pacific and the Oriental Tethys, with nearly identical species ranging from the far north to the south temperate region in Argentina, and from the Mediterranean eastward to Timor and westward to California.

In the Middle Jurassic similar geographic relations continued, with clear evidence of a warm climate from Franz Joseph Land to Madagascar and Argentina, and no division into climatic zones. Cycads flourished on the land, and corals built reefs in the seas. This was too good a state of affairs to last long in this world, and there soon came a change. The Central American portal was closed, as was that between the Pacific and the Tethys. The Arctic Sea became the center of dispersion of a boreal fauna, which made its way down to Russia on one side, and to California and Mexico on the other. This arrangement of the geographic provinces reminds us strongly of that during the Triassic epoch of Pseudomonotis subcircularis and it continued into the Cretaceous period.

The Upper Cretaceous is signalized by the closing of the Bering portal, and the wide distribution of a tropical fauna from the Indian waters westward to the Mediterranean waters, and eastward through Japan, around the North Pacific shore-line to California. This same fauna was also distributed from the Mediterranean westward through the Poseidon Sea to Mexico, and southward into South America. And