13 



America, l^nt the}^ are not sufficiently well determined to be 

 of service in this connection. Others have been rej)orted from 

 supposed Jurassic strata, which are now known to be Creta- 

 ceous. It will thus be seen that, although reptilian life was 

 especially abundant during the Triassic and Jurassic periods, 

 but few bones have been found. This is owing in part to the 

 character of most of the rocks then formed, which were not 

 well fitted for preserving such, remains, although admirably 

 adapted to retain foot-prints. 



During the Cretaceous Period, Eeptilian life in America 

 attained its greatest development, and the sediments laid down 

 in the open seas and estuaries were usually most favorable for 

 the preservation of a faithful record of its various phases. With- 

 out such a perfect matrix as some of these deposits afford, many 

 of the most interesting vertebrates recently brought to light 

 from this formation would probably have remained unknown. 

 The vast extent of these beds ensures, moreover, many future 

 discoveries of interest. 



In the lowest Cretaceous strata of the Rocky Mountain 

 region, the Dakota group, part of which at least represents 

 the Wealden of Europe, remains of Chelonia^ or Turtles, Croc- 

 odiles, and Dinosaurs occur, the last being especially abun- 

 dant. The Chelonia^ although known from the Jurassic of 

 Europe, here appear for the first time in American rocks. 

 Some of the earliest forms are allied to the modern genus 

 Trionyx. In the higher Cretaceous beds, some Chelonians of 

 enormous size have been found. They belong to the genus 

 Ailantochehjs, which has the ribs separate, as in tbe existing 

 Spharyis, and presents other embryonic characters. A few 

 genera appear to be related to the modern genus Ghelone. The 

 remaining Cretaceous species were mostl}' of the Emydoid 

 type ; and others were related to Chelydra. The more important 

 genera of Cretaceous Chelonians known from characteristic speci- 

 mens are, Atlanioclielys {Protostega), Adocus, Bothremys, Compse- 

 mys, Pkisiomemis, Osteopygis^ Propleura, Lytoloma, and Taphros- 

 pliys. Most of these genera were represented b}' several species. 



