1 66 5. W. WILLISTON 



tion of the microsaurs had reached the extraordinary extent of snake- 

 like hmbless forms. 



In addition to these two types of land animals we have two others 

 which either persisted from unknown ancestors or made their advent: 

 the temnospondylous type of amphibians from which the mammals 

 eventually arose, and the stereospondylous type which terminated 

 in the gigantic labyrinthodonts of the Upper Trias, the only group 

 of the Pennsylvanian air-breathing vertebrates which we may say with 

 certainty has left no modern descendants behind them. However, 

 till near the close of the Pennsylvanian we have no knowledge of 

 anything distinctive in the American land-vertebrate fauna. There 

 was nothing strikingly peculiar to either eastern or western continent, 

 so far as our knowledge yet extends, and some of the forms, indeed, 

 are almost if not quite identical generically. And the only possible 

 explanation of this homogeneity of types is freedom of communica- 

 tion and migration, the persistence and wide extent of like climatic 

 and freshwater conditions that would permit, for instance, the migra- 

 tion of snake-like forms of small size from Ohio to Ireland and Bohe- 

 mia without material modification in structure. 



However, either the divisional lines between the Pennsylvanian 

 and the Permian have been placed too high, or else, it seems to me, 

 evolution among the vertebrates was more rapid in America than else- 

 where near the close of the period. As a continent I believe that the 

 land of North America was absolutely and continuously isolated, so far 

 as the intermigrations of land forms was concerned, from some time 

 before the close of the Pennsylvanian till well into Triassic times, as 

 they reckon them in Europe. Of the Permian vertebrates by far the 

 richest and most varied fauna known is that of America, and especially 

 that of Texas and Oklahoma. Professor Case has recently presented 

 what evidence he could for the Permian age of this fauna and has 

 admittedly failed in proving anything save its utter isolation, and 

 from the evidence we yet have no one can do better than he has done. 

 The fauna was literally sui generis and I may almost say sui ordinis. 

 But two or three genera of two types out of the scores of genera known 

 from these regions can be correlated as showing resemblances — 

 family resemblances I mean — with foreign forms. And both of these 

 types had made their appearance, admittedly now here in America, 



