16 



a. ¥. MAÏTllKW ON THl'; DIFFUSIOK ETC. 



Ordovician, and a double vertical line marks oil' the purely American I'auuas; The 

 presence of Triarthrus in the right-hand column is really due to the American area having 

 been invaded by an Atlantic or deep-water fauna (that of the Utica slate) carrying this 



Table to show the Migkation of Genera from North America to Europe in 



Cambrian and Ordovician Times. 



