36 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. [Yol.Y. 



of the period until the epoch of the Mobrara, and then gradually shal- 

 lowed until the elevations of the bottom began to divide the waters. 

 The closing scenes of this great period were enacted amid a labyrinth 

 of lagoons and lakes of brackish and fresh water, whose deposits form 

 the beds of the Laramie epoch. 



The fauna of the deep-sea epoch, the Mobrara, is the best known. 

 Here the remains of Pytlionomorpha constitute its prevailing character- 

 istic, while Elasmosaurus and Polycotylus, with but few species, represent 

 the niunerous Sauroptenjyia of Europe. Crocodiles were apparently 

 wanting, while turtles and a peculiar group of Fterosauria were only 

 moderately abundant. The fish fauna was very rich and varied. Here 

 the Saurodontidce, like the Molluscous family of the Eudistes, appeared, 

 and as soon disappeared, accompanied by the peculiar form, EHsicMliej 

 and the family of Stratodontidce. The genera of Mount Lebanon, Lepto- 

 trachelus and Spaniodon, occur in this bed in Dakota; but the closest paral- 

 lelism is exhibited with the Lower Chalk or Turonian of Western Europe. 

 The general /a aes of the reptilian fauna is that of the Lower Chalk, and 

 there is little doubt that several genera are identical in the two conti- 

 nents, e. g. Elasmosaurus. The apparent peculiarity of the Chalk in 

 America is the abimdance of forms (four genera) of Fytlionomorpha with 

 numerous species, while but two genera have yet been found in Europe, 

 and the presence of birds, with biconcave vertebrae, and teeth. This in- 

 teresting type, which was first discovered by Seeley in the genus named 

 by him Enaliornis, and afterwards found by Marsh to possess teeth, has 

 been found at a lower horizon in England, the UpiJer Greensand. But 

 in England, France, and Westphaha occur the genera of fishes above 

 mentioned, as Portlieus, Iclithyodectes, Saurodo7i, Saurocephalus, Eri- 

 sicJitJie, Eonpo, FachyrMzodus, Enchodus, LeptotracJielus, etc. This close 

 relationshix) of the horizons i^ermits an identification, and it is the first 

 instance which appears to me to be susceptible of satisfactory demon- 

 stration. 



The next horizon of the Cretaceous which has yielded many verte- 

 brate remains in ISTorth America is the Fox Hills formation (including 

 the Fort Pierre bed). Here the genus Mosasaurus appears in America, 

 and is accompanied by the earliest crocodiles with proccelous vertebrae, 

 and by numerous marine turtles which partake of the characters of both 

 Chelydridw and Cheloniidce, wiiich I have called the Fropleuridw. Beryx 

 appears first here in America. The predominant genus of fishes is En- 

 cJiod.us, and the principal Dinosauria are Lodaps and Sadrosaurus. This 

 horizon has been x)arallelized with the Maestricht of Europe, and sev- 

 eral genera are common to the two beds ; such are Mosasatirus and 

 Enchodus. The genus Sadrosaurus, and the family of turtles I have 

 called the Adocidw, remain undiscovered in Europe ; hence the identity 

 of faunge cannot be estabhshed. 



The lacustrine beds, or summit of the American Cretaceous series, the 

 Laramie of Hayden, present the remains of a populous fauna and a rich 



