398 S. W. WILLISTON 
lingerer, which had apparently disappeared in Europe, and the first 
of the slender-nosed crocodiles of olden type, their first appearance 
here after their last records from the eastern continent. And with 
them appears for the first time a new type of dinosaurs, the armored 
polacanthids (Stegopelta) which had appeared in Europe in the 
Wealden, but which is unknown from the earlier deposits of America 
among all the vast numbers of dinosaurs. With the close of the Ben- 
ton and the beginning of the Niobrara, we find the first appearance 
of distinctive American types of air-breathing vertebrates since the 
decay of the Permian fauna, save the thalattosaurs of the Pacific 
Trias, in the large marine turtles (Protostega) and the duck-billed 
dinosaurs (Claosaurus). And what is very interesting is the first 
appearance of the scaled reptiles, the mosasaurs, in America. But 
the mosasaurs had already reached a high degree of importance in 
the east and perhaps in the south. They appear here suddenly 
without any such premonitions as are found in southern Europe, 
long after their appearance there. Although marine animals, they 
live near the shores and doubtfully ever braved the oceans; they 
must have followed the land. The birds, too, now are numerous and 
of considerable diversity of form; and the pterodactyls swarmed the 
seas, pterodactyls which had gradually been evolving in Europe till 
they had reached almost or quite the American specialization in the 
Cambridge Greensand. What was the cause of their delay in reach- 
ing this continent ? Certainly not our lack of knowledge of the faunas, 
for I believe that we can say with tolerable certainty that no ptero- 
dactyls were in existence here till the time of the Colorado Cretaceous, 
certainly none of the Cretaceous type which began in the Wealden 
of Europe. The plesiosaurs, on the other hand, have taken on 
specializations which, notwithstanding their supposed freedom of 
migration, indicate comparative isolation from the European forms, 
for not a single genus is identical, and, save possibly Platecarpus, 
there is not a single genus of mosasaur quite identical with those of 
the European fauna. Unfortunately we know little of the land 
animals of this epoch, but altogether I think we are justified in saying 
that the freedom of communication between European and American 
land vertebrates was somewhat restricted. 
During the times of the Fort Pierre and Laramie, inclusive of the 
