REP'nLIA. 



55 



Three genera are commonly recognized in the living forms, 

 Gavialis and Crocodilus, of which there are several species, the 

 latter in every continent, and Alligator, with a single species. 

 Some authors make Caiman {Jacare) a separate genus, and 

 Tomistoma, of Borneo, still another. 



The order is represented in the Triassic of both Europe and 

 America by Belodon, with biconcave vei'tebrae, and the line of 

 descent was continued throughout Teleosaurus in the Jurassic, 

 which group became extinct in the Upper Cretaceous, Diplo- 

 saurus of Upper Jurassic was a link between the Triassic and 

 modern species. The Gavial type was well established in the 

 Upper Cretaceous, in Thoracosaurus. Cope enumerates five 

 genera and fourteen species of the order from the American 

 Cretaceous. 



The modern type with procoelian vertebrae reaches back only 

 to the Upper Cretaceous. Gavials and crocodiles abounded in 

 the Eocene. The Alligators, a specialized form of Crocodilus, 

 appeared in Europe in the Tertiary, but are not found in America 

 until later time. The eai-lier forms were doubtless marine. 



No. 82. [248j Thoracosaurus neocesariensis, DeKay. 



(Crocodilus clavirostris, Morton). 



Skull (cast). This is cue of the earliest Crocodilians witli cup and ball 

 vertebrae. This specimen was found in the Cretaceous beds of New Jersey, 

 and is in the Museum of the Academy of Natural Sciences at Philadelphia. 



Size, 25 X 13. 



No. 83. [249] Crocodileimus robustus. 



Skeleton, on slab (cast). Original from 

 the Lithographic Limestone (Middle Oolite) in 

 the Department of Ain, France, and now in 

 the Museum of Natural History at Lyons. 



Size, 18 X lO: 



