The ichthyosaurs began their existence, so far as we now know, about the middle of Triassic times and continued to near the middle of Upper Cretaceous, when they disappeared forever from geological history. As we have seen, however, the earliest forms that we know were true ichthyosaurs in all respects, though more primitive than the later ones, indicating a long previous existence of which we yet have no knowledge. Their remains have been found widely distributed in Triassic rocks of Europe, Spitsbergen, Australia, and North America. During the Jurassic period they lived in great numbers and variety throughout the region that is now Europe. In North America the only marine rocks of this period that we know of have yielded numerous remains. These American ichthyosaurs were, however, among the most specialized of all ichthyosaurs—the culmination of their development. They were originally named _Sauranodon_ in the belief that they were toothless, but in recent years their teeth, small and numerous, have been discovered. And the genus seems also to be identical with one previously named from the Jurassic of Europe called _Ophthalmosaurus_. The last known remains of ichthyosaurs have recently been found in the Benton Cretaceous of Wyoming. Scanty remains of ichthyosaurs are also known from Australia and New Zealand. Why the ichthyosaurs should have gone out of existence before the plesiosaurs and mosasaurs did, one cannot say; possibly their stock had grown old and feeble. CHAPTER IX PROGANOSAURIA MESOSAURUS There is some doubt whether those little creatures of Paleozoic times, to which some years ago the late Professor Baur gave the ordinal name Proganosauria, are really entitled to so much distinction among reptiles. The question of their rank has been much disputed for the past twenty years without any positive conclusion. Nor were they wholly aquatic in habit, though they did possess many aquatic adaptations. That they were skilful and fleet swimmers, and capable of rapid evolutions in the water is quite certain, and, as the oldest known water reptiles, they are of more than passing interest. [Illustration: FIG. 60.—_Mesosaurus_; life restoration, after McGregor, the posture of hind leg slightly modified.]