PILSBRY: non-marine MOLLUSCA of PATAGONIA. 615 



advocated by Dr. von Ihering from the evidence of the fresh-water fauna, 

 is essential to any rational explanation of the distribution of land and 

 fresh-water mollusks. 



The following South American groups are common to the tropical 

 African radiation center. Terrestrial groups are marked with an asterisk (*). 

 ^ Strcptaxidcc (fig. 21). 

 '*■ AchatinidcB (fig. 20). 

 * VeronicellidcE. 

 Pla)iorbince (South American Plaiiorbis and Plesiophysa close to African 



Planorbis and Isidora). 

 AmpullayiidcB (genera with corneous opercula, fig. 22). 

 Melaiiiidce (nearest to African forms). 

 Mutelidce (fig. 23). 

 EtJieriidce (fig. 24). 

 SphcEviidcc, of the genus Eupera. 



Such of these groups as are represented in the West Indies and sub- 

 tropical North America, have evidently, from their distribution, relation- 

 ships and the greatly diminished number of genera and species, been 

 derived from South America, rather than from some common source, such 

 as Archhelenis. None of them are present as Mesozoic or Tertiary 

 fossils in North America,^ and nearly all of them are likewise absent from 

 European deposits. 



There is a good deal of evidence that most of the above groups arose 

 in the eastern hemisphere and migrated westward, and little evidence or 

 none that any of them moved in the opposite direction. 



The Streptaxidcc, AcJiatinidcE, AmpullayudcB, Melaniidce and Mutelidce 

 are far more diversified in Africa than in South America. It seems that 

 several Gondwana subfamilies were not present in the fauna of Archhelenis, 

 or at least did not extend so far west as to reach South America, though 

 they must date back at least as far as some other subfamilies which are 

 represented both in South America and Africa. 



The primitive stock of fresh-water mussels seems to have early split into 

 two phyla : the one, Mutelidcs, evolving in the south, on the Gondwana- 

 Archhelenis continent, the other, Unionidce, in the north, in North America 



* The Planorbinm are found on all continents, but those of South America are obviously most 

 closely related to the African forms. 



