174 S. W. WILLISTON 



eastern continent. The Upper Cretaceous again shows a belated 

 arrival on the western continent of eastern types, after their advent 

 or even disappearance there. With the exception of certain Triassic 

 marine types, we have no distinctively American Mesozoic groups of 

 air-breathing vertebrates, until we reach the Benton, Niobrara, and 

 Pierre Cretaceous, all indicating a continued, but possibly restricted 

 intermigration between the eastern and western continents during 

 the whole of Mesozoic times. In which way did these migrations 

 occur ? That the communication between the two continents in Penn- 

 sylvanian time may have been by way of the north Atlantic region 

 is not at all improbable. Indeed, taking into consideration the close 

 relationships known to exist between the European and American 

 type of this period, closer perhaps than existed at any subsequent 

 time during the Mesozoic, this more direct way of communication 

 would seem very probable. 



On the other hand, the very close relationships existing between 

 the species of the Proganosauria, hitherto found only in South x\mer- 

 ica and Africa, one genus of which is exclusively American while 

 the other genus, Mesosaurus, according to McGregor's recent obser- 

 vations, is represented in both continents by closely allied species, 

 would suggest a close land communication between the two conti- 

 nents during early Permian times at least. That Mesosaurus may 

 have reached the two continents, Africa and South America, by the 

 long, roundabout way of the north Atlantic, is hardly possible, for 

 the same freedom of communication would have opened up North 

 America to the ingress and egress of European and American forms. 

 It would seem altogether probable, then, that there not only was a 

 free communication between Africa and South America in Permian 

 times, but that also the communication between North and South 

 America was closed during the same interval, though of this we cannot 

 be at all sure till we know more of the South American Permian fauna, 

 which, so far, lacks every distinctive form peculiar to North America. 



Whether or not the communication between North America and 

 the eastern continents was by way of the north Atlantic, it is quite 

 probable that there was more or less free communication during part 

 or all of the Mesozoic timfe between North and South America, proof 

 of which is seen in the dinosaurs, mosasaurs, and crocodiles, some of 

 them, according to competent observers, identical generically even 



