400 S: W. WILLISTON 
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 
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 Amer- 
ica and Africa, one genus of which is exclusively American while 
the other genus, Mesosternum, 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 Mesosternum 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 
