FAUNAL RELATIONS OF EARLY VERTEBRATES 173 



waxed and grew mighty. A new type for America of terrestrial 

 turtles appeared. The polacanthid dinosaurs, long since unknown 

 in Europe, continued to the very close (Paleoscincus). The mosa- 

 saurs present a European genus (Mosasaurus), but one that was 

 most certainly developed here in America, and emigrated. Finally 

 at the close a new type of reptiles (Choristodera), with marked rhyn- 

 chocephalian affinities, appears, continuing on into the Tertiary, both 

 here and in Europe, in forms almost generically identical; and the 

 same may be said of the American crocodiles (Thoracosaurus) which 

 reappear in Europe in the early Tertiary, with scarcely any differences. 



And all these facts indicate conclusively a continued intermigration 

 between the eastern and western continents of land animals, with 

 possibly some less freedom during late Cretaceous times. 



To summarize: The Pennsylvanian fauna has nothing distinctive, 

 at least till near the close; there must have been a continuous and 

 free interchange of land animals with the eastern continent till near 

 the close. Before its close, it had already diverged and certain true 

 reptiles had appeared. Before the beginning of Permian times an 

 interruption of migration occurred, producing a complete and con- 

 tinuous isolation of the Permian American fauna. With the close 

 of these times a long interval elapsed, during which physical condi- 

 tions were almost uniform over a large part of the Rocky Mountain 

 area at least; during which interval we have no records of land or 

 freshwater life, but which is represented in part by marine forms of 

 remarkable character, possibly in part derived from American ances- 

 tors. With the reappearance of land forms in the Upper Trias we 

 find certain evidence of free migrations again, with the closest rela- 

 tionships between eastern and western forms, none of which could 

 have been derived, immediately at least, from the known American 

 Permian types. The marine vertebrates of the Upper Jurassic, 

 the next American air-breathers of which we have any knowledge, 

 indicate an advance in specialization over the contemporary forms 

 from the eastern continent, but they also indicate a continued migra- 

 tion of the aquatic forms at least. With the land forms again appear- 

 ing at the close of the Jurassic and in the Lower Cretaceous, we find 

 strong evidence of a community of faunas, but with a striking absence, 

 hitherto, of some of the smaller forms known from earlier times in the 



