Since geology entered the order of exact sciences, it has been acknowledged by all those, who thoroughly studied it, that the history of the earth is to be divided into three great periods, which have been called, after the faunas they contain, Palæozoic, Mesozoic and Cainozoic. Indeed it could not escape the eye of the palæontologist, that between the marine faunas of the Permian period and the Trias on one side, and the Chalk sea and the Eocene period on the other, there is a greater difference than between the animal world of any of the other periods. In the transition-period between Permian and Trias, Corals, Cephalopods, Reptiles, etc., underwent an astonishing change; thousands of forms seemed to have perished, thousands of new ones appeared; in the succeeding time, shortly after the commencement the fauna of the Mesozoic seas showed, on the whole, quite another stamp, than that of the Palæozoic seas. A quite undisturbed development seemed to have reigned from Silurian to Permian, and there seemed to have been a sudden interruption of that development during the Permian period. Again, there followed an era of the most quiet and peaceful evolution during the Triassic, Oolitic and Cretaceous periods, where the connection between the several marine faunas seems to have been an uninterrupted one; then, at the end of the Cretaceous period, the marine fauna underwent a new change. The dawn of a new time (Eocene) begins, thousands of forms again disappear to be replaced a second time by new ones, as had occurred during the Permian period. Again, the Cephalopods seem to have been the class against which a war of annihilation was directed, and in their place the Pelecypods and Gasteropods take the leadership in the new era. The Enaliosaurians having reached a high development die out entirely and the Crocodilians retire to the rivers. The Secondary fauna gives way to the Tertiary, bearing a thoroughly new stamp.