14 



and the individuals were numerous. No land Tortoises have as 

 yet been found in this formation. In American Tertiary depos- 

 its, Chelonians are abundant, especially in the fresh-water beds. 

 They all show uear 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 con- 

 trasted 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 

 Sti-eptosauria, 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, Cimolio- 

 saurus, Discosaurus (fflasmosaurus), and Pliosaurus. The num- 

 ber 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 Eeptiles 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 begin- 

 ning to emerge, these ancient " Sea Serpents" abounded ; and 

 many were entombed in its muddy bottom. On one occasion, 



