Page:Popular Science Monthly Volume 76.djvu/486

482 forms, habituated to lower temperatures. When the amelioration of the earth's climate took place, near the beginning of the Mesozoic era, they found a free field on the coasts, and at once took possession. In the epoch of the Middle Triassic they had already become widely distributed, but as yet had formed no known reefs.

The distribution of the cephalopods in time shows a strong contrast to that of the corals. There is an unbroken genetic series of ammonoids and nautiloids from the Coal Measures, through the Permian, and extending into the Lower Triassic, several genera ranging through the interval. This does not necessarily mean that the cephalopods were hardier, for they probably were not. But they were very widely distributed, and must have lived on in some region, or regions, where great catastrophe had little or no effect, and by their superior facility in locomotion got back into the regions affected by glaciation, when the temperature of the seas had risen again.

Since corals are wholly unknown in the Lower Triassic, and since the flora of that epoch is as yet little known, it is not possible to determine the temperature of either the land or the water. It is, however, certain that the oceanic temperature in India, in western America and in northern Siberia, was the same, for there is a remarkable similarity of the cephalopod faunas in all three regions.

It is also known that in the Permian and the Lower Triassic a dry climate prevailed over large areas, for products of desiccation, such as gypsum and saline deposits are common in many parts of the world, and even in regions that are now rainy, as in western Europe.

In the Upper Triassic there are great limestone masses and coral reefs in the Alps, the Himalayas and in California, with many species common to the three regions. Certainly the epoch of the Tropites subbullatus fauna was tropical up as far as Shasta County, California, for there reefs of Astræidæ are extensive. We may even be justified in assuming that the isotherm of 74° F. extended that far north. Also in the Blue Mountains of northeastern Oregon there are coral reefs in the Upper Triassic, but no Astræidæ were found in them, only extinct genera. This outlying occurrence may correspond to the isotherm of 68° F., in which now corals may form reefs, but Astræidæ can not flourish.

After the formation of the coral reefs in northern California and Oregon the facies changed suddenly from limestones to clay shales, and with this came an abrupt change in the marine fauna. The Indian types of cephalopods disappeared entirely, and in their stead came in a fauna of which the home seems to have been the boreal region. Pseudomonotis ochotica was the commonest species in this fauna, and was