172 5. W. WILLISTON 



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 

 New Jersey and Judith River faunas, we get some notable, though 

 very dilatory appearances of European forms, the first land scaled 

 reptiles, the first salamanders, and, with them, the first of the modern 

 type of crocodiles, allied to the Borneo gavials. And with them also, 

 the very much belated long-headed crocodiles of ancient type gave 

 up the ghost, while the duck-billed and horned dinosaurs and the 

 marine turtles, all distinctively American forms, the most distinctive 

 of American Mesozoic vertebrates save thalattosaurs that we have, 



