PALEOGEOGRAPHIC MAPS OF NORTH AMERICA 405 
south warm. ‘The vertebrates known from Nova Scotia to Texas 
appear to have lived in the more genial regions and to have had no 
communication (unless closely following the Pennsylvanian) with 
Europe or South America, although the latter was connected with 
Africa by some southern route. The barriers to intermigration in 
the north may have been marine waters (North Atlantic) and cold 
climate (Alaska-Siberia).* 
1 My thanks are due to Dr. S. W. Williston for discussion of the evidence regarding 
vertebrates.—B, W. 
