PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 633 



of the egg-capsules of AmnicoKda. It would be absurd to suppose that 

 they could ever be spread by aquatic birds. 



Such evidence as we have favors the view that the connection of South 

 America with Antarctica was transitory, hence taken advantage of by but 

 few genera of mollusks, all belonging to families richly developed in the 

 South American center. So far as non-marine mollusks show, the migra- 

 tion from South America of a few species belonging to three families will 

 account for all the common elements in the austral lands of the two 

 hemispheres. The strong generic differentiation of all the common 

 austral groups, with the exception of Potamopyrgits and probably Diplo- 

 don, indicates that the connection was of considerable antiquity, probably, 

 as Dr. Ortmann holds, not later than Eocene. 



Summary.- -The South American molluscan fauna is traceable to two 

 sources : an ancient southern continent lying across the south Atlantic 

 and enduring from at least Palaeozoic to near the end of Cretaceous time, 

 and to Miocene and Pliocene to recent connections with the middle 

 American area. Antarctica was not an evolution center for mollusks, but 

 there is strong evidence that a few groups passed by the Antarctic route 

 to Australasia. " Archiplata," owing to its physical diversity from the 

 Brazilian and Colombian areas, has became a Tertiary evolution center 

 for a few groups of Brazilian origin. 



Middle America (Antilles + Central America and part of Mexico) has 

 the characteristics of an old evolution center of the northern faunal 

 group, its primitive fauna coming from the north, and now showing phylo- 

 gerontic features ; a later (probably late Cretaceous) element was derived 

 from the East Asiatic fauna. Both elements contributed, during the last 

 half of the Tertiary, to the South American fauna, and received immi- 

 grants in return. 



The " Nearctic Realm," so far as the genesis of its faunas is concerned, 

 is composite. 



