VERTEBRATE LIFE IN AMERICA. 521 



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 l type ; and others were related 

 to Ghelydra. The more important genera of Cretaceous Chelonians 

 known from characteristic specimens are Atlantochelys (Protoste- 

 ya), Adocus, Pothrernys, Compsemys, Plastomenus, Osteopyyis, Pro- 

 pleura, Zytoloma, 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 Plesiosaicria, however, is well repre- 

 sented, but mainly by forms more nearly related to the genus Plio- 

 saurus 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 Vimo- 

 liosaicrus, Discosaurus (Elasmosaurus), and Pliosaurus. The num- 

 ber of species is comparatively few, and none are known above the 

 Cretaceous. The important suggestion of Gegenbaur, that the Hali- 

 satiria, 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-ser- 

 pents" 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 raon- 



the largest of all turtles, and have the body covered with a thick coriaceous skin instead 

 of a hard shell. 



1 Emys, a genus of small land and fresh-water tortoises. 



