CLASSIFICATION OP OLEACINIME. xiii 



The Oleacinida:, while quite distinct, have much in common 

 with the family Rliytididce. The form and position of the 

 kidney and the shape of the shell are the chief characters 

 separating the two families. No other existing group is 

 closely related. 



Origin. The family most closely related to the Oleacinida; 

 is an exclusively Old World group the African and Austra- 

 lasian Rhytididce. No fossil Oleacinidcc older than Pliocene 

 are known in America, but as yet we know no brackish or 

 fresh-water deposits of tertiary or rnesozoic age in the 

 American tropics, excepting the Oligocene Pebas beds, in 

 which such shells would occur. In Europe, on the other 

 hand, the Oleacinida have a continuous geological history 

 from the present day back to the Cretaceous. 



One Cretaceous species is from southern France (Pro- 

 vence), while in the Eocene and Oligocene, numerous forms 

 occur as far north as the Isle of Wight. Miocene species are 

 found in Germany, Bohemia and Italy. It is probable there- 

 fore that all known European Oleacinida: belong to a single 

 stock of which Poiretia algira and its allies are the sole sur- 

 vivors. 



From Jurassic to late tertiary time, the European localities 

 are believed by those best qualified to judge Neumayr, Koken 

 and others to have been separated by a " Mediterranean" 

 sea from Africa. Their faunas are believed to have been in 

 part insular, in part perhaps continental on a large " Scan- 

 dinavian" land, which in the cretaceous extended probably 

 as far southwest as Spain (See KOKEN, Die Vorwelt und ihre 

 Entwickelungsgeschichte, plate I). It is extremely likely 

 that from the Spanish area a peninsula extended southwest 

 at least as far as Madeira. Later this land was more broken 

 up. I believe on faunistic grounds that the common elements 

 of the late Cretaceous and Eocene European faunas and the 

 Brazil-Ethiopian continent, must be traceable to an earlier 

 connection, or to a Cretaceous connection between Europe 

 and Africa at present unrecognized by geologists. A later 

 (Eocene) connection from Europe southward is improbable. 

 Whether the Oleacinida: arose in the north, on the Scandina- 



