Page:Popular Science Monthly Volume 12.djvu/539

Rh and presents other embryonic characters. A few genera appear to be related to the modern genus Chelone. The remaining Cretaceous species were mostly of the Emydoid type; and others were related to Chelydra. The more important genera of Cretaceous Chelonians known from characteristic specimens are—Atlantochelys (Protostega), Adocus, Bothremys, Compsemys, Plastomenus, Osteopygis, Propleura, Lytoloma, and Taphrosphys. Most of these genera were represented by several species, and the individuals were numerous. No land-tortoises have as yet been found in this formation. In American Tertiary deposits, Chelonians are abundant, especially in the fresh-water beds. They all show near affinities with modern types, and most of them can be referred to existing genera. In the Tertiary lake-basins of the West, land-tortoises are very numerous, and with them are many fresh-water forms of Trionyx and allied genera.

A striking feature of the American Cretaceous fauna, as contrasted with that of Europe, is the almost entire absence in our strata of species of Ichthyosaurus and Plesiosaurus, which abound in many other regions, but here seem to be replaced by the Mosasaurs. A few fragmentary remains have indeed been referred to these genera, but the determination may fairly be questioned. This is more than true of the proposed new order Streptosauria, which was founded wholly on error. The order Plesiosauria, however, is well represented, but mainly by forms more nearly related to the genus Pliosaurus than to the type of the group. These were marine reptiles, all of large size, while some of them attained vast dimensions. So far as at present identified, they may be referred to the genera Climoliosaurus, Discosaurus (Elasmosaurus), and Pliosaurus. The number of species is comparatively few, and none are known above the Cretaceous. The important suggestion of Gegenbaur, that the Halisauria, which include the Plesiosaurs, branched off from the fishes before the Amphibians, finds some support in American specimens recently discovered.

The Reptiles most characteristic of our American Cretaceous strata are the Mosasauria, a group with very few representatives in other parts of the world. In our Cretaceous seas, they ruled supreme, as their numbers, size, and carnivorous habits, enabled them to easily vanquish all rivals. Some were at least sixty feet in length, and the smallest ten or twelve. In the inland Cretaceous sea from which the Rocky Mountains were beginning to emerge, these ancient "sea-serpents" abounded; and many were entombed in its muddy bottom. On one occasion, as I rode through a valley washed out of this old ocean-bed, I saw no less than seven different skeletons of these