No. 391.] BIPOLARITY OF MARINE FAUNAS. 591 
sea (Ortmann, von Ihering, Thompson, Chun, Ludwig); (2) con- 
nection along the western shores of the continents, mostly con- 
nected with a descending of the respective forms into deeper 
water (Ortmann, Bouvier, Thompson, Ludwig). 
It is possible that by these ways cases of true bipolarity may 
develop, provided these connections become discontinued. The 
writer has explained a true case of bipolarity (Crangon) by one 
of these ways. But, on the other hand, it is also possible that 
bipolarity is to be explained by the Pfeffer-Murray theory in 
some cases by former conditions of the earth’s history, espe- 
cially those existing at the beginning of the Tertiary period. 
Yet we do not know any concrete case of this kind, and we 
must wait for further investigation to show whether bipolarity 
as a relic of older times is realized in the geographical distribu- 
tion of any marine animals. 
