438 



Tri:ST IXDIAN MOLLUSES—SIMPSOX. 



Land and fresh-water mollusJcs i Jniimj the Greater Antilles and the continent. 



*Thi8 species is distribnted over nearly all the United States and as far south as Honduras. 

 Prime states (American CorbiculidiB, p. 76), that P. conxdiiiiiiiiic.nn. Prime, of Cuba can scarcely be 

 separated from i'. (li^f/iiwrn Hald., and on carefully eompariug authentic specimens in the National 

 Museum I believe them to be the same. 



I am not prepared to believe that so extensive a relationship — the 

 exchange of so many genera, subordinate groups, and species — could 

 have been brought about merely by ocean currents and winds. Since 

 the gulf stream was turned into its present course — probably during 

 the later miocene, when the Isthmus of Panama was elevated — its tend- 

 ency would be to sweep any species that might fall into it, from the 

 Antilles or the continent, up into the Gulf of Mexico, and away from 

 either shore. The j)revailing winds of the region have no doubt been 

 from the eastruorth-east in the past, as in the present, and would favor 

 the landing of Antillean species in Yucatan, though their effect would 

 be largely neutralized by the current. We find that very nearly as 

 great a migration has taken jjlace from the mainland to the archipel 

 ago as in an opposite direction. The depth of the Yucatan Channel 

 would seem to preclude the likelihood of a former landway running 

 west from Cuba, but the presence of Sfreptostyla, with eight species 

 scattered through Cuba, Haiti, aud Puerto Rico, and Volutaxis with two 



