614 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



Succineidce. World-wide, on all continents and islands. 

 Chilinidce. Autochthonous. 

 Amnicolidce. On all continents. 



The ancestral stocks of these families probably formed the earliest 

 fauna of non-marine mollusks in South America. Some of the families 

 are known, in other regions, to have been established in Palaeozoic times ; 

 and it is likely that as early as that they were already found in South 

 America. All of them are sharply isolated groups. 



The Circinariidce seem to have remote affinity to Rhytididce and Strep- 

 taxidce both of which evolved in the southern hemisphere of the Old 

 World. In North America this family is probably intrusive, being repre- 

 sented by a single genus also found in South America. 



The Strophocheilidce have relations though not close with a series 

 of genera (Acavidce] now found in south Africa, Madagascar, the Sey- 

 chelles, Ceylon, Moluccas, Australia and Tasmania. The radiation of 

 this scattered group from the Palaeozoic Gondwana continent of Neumayr 

 seems a reasonable, in fact the only tenable, hypothesis. 



The BulfmuKda have descended from the Holopod stock, probably 

 also of Gondwana Land, since we have no evidence of any other ancestry. 



Endodontidce and Succineidce are world-wide groups, even on the most 

 remote islands. Their early presence in South America is therefore likely. 



The Chilinidce represent an isolated branch of the primitive Basomma- 

 tophora. No scrap of evidence has been brought to light to show that 

 they ever existed elsewhere than in South America ; and at present we 

 have every reason to believe that there they invaded fresh water from 

 the sea. 



The Pectinidens group of Lymnceidce and the Ancylidce are evidently 

 traceable to some very early radiation. Adequate data upon the soft 

 anatomy and relationships of the forms of the southern hemisphere do not 

 now exist. 



II. THE ARCHHELENIC GROUP. 



Families which for the Greater Part are Represented in the 



Tropical African Fauna by afar Greater Diversity of 



Forms than in the South American. 



The hypothesis of an Africo-Brazilian continent of Palaeozoic and Meso- 

 zoic times, first sketched out on purely palasontological grounds, and ably 



