170 5. W. WILLISTON 



of reptiles, the Synaptosauria. Between the horizons yielding Per- 

 mian fossils, whether vertebrate or invertebrate, and that affording 

 these Keuper Triassic animals, there are, in both Kansas and the 

 Lander region of Wyoming, at least a thousand feet of continuous, 

 conformable, uninterrupted, and homogeneous deposits of red sand- 

 stones, deposits utterly barren of all animal or plant remains. I have 

 asked geologists in vain what such deposits mean. One thing they 

 do mean, for the Rocky Mountain region at least — continuous and 

 uniform physical conditions. What became of the Permian verte- 

 brates during this interval we cannot say, for, as I have said, there is, 

 I believe, not the slightest trace of them or their descendants in the 

 land fauna. And from the east, as also from the west, we get before 

 the close abundant evidence of dinosaurs and aetosaurs; and a peculiar 

 type of possible mammals, from Carolina. 



Again comes a most lamentable gap in our knowledge of land 

 vertebrates in America, that of the Lower and Middle Jurassic. With 

 the Upper Jurassic marine beds, come in the most specialized of the 

 ichthyosaurs and highly specialized plesiosaurs and a single frag- 

 mentary specimen of a crocodile, the first from the American continent. 

 Both the ichthyosaurs and the plesiosaurs show such high evolution 

 that we must admit their recent migration from Europe, where indeed 

 a closely related ichthyosaur had anticipated our form and the plesio- 

 saurs had long been known. 



With the close of the Jura a rich land fauna appears in the Morri- 

 son beds, rich but not varied, composed almost exclusively of dino- 

 saurs, dinosaurs big, dinosaurs small, carnivorous, herbivorous, walk- 

 ing, running, almost flying dinosaurs, of high and low degree, but 

 among them all not a single type that is distinctively American, not 

 one that is not, prior to this time or as a contemporary with it, known 

 from the eastern continent. Morosaurus mimics Cetiosaurus, Camp- 

 tosaurus Iguanodon, Stegosaurus Omosaurus, Allosaurus Megalosau- 

 rus, etc. We are confident then that during Morrison times there 

 was freedom of migration between the eastern and western continents, 

 so free that nothing distinctive of our fauna had been developed 

 through isolation. Here now we find for the first time meager repre- 

 sentatives of the first turtles, of a single type, which had been known 

 on the eastern continent since Middle Triassic times, almost the first 



