PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 



623 



the Caribbean Islands on the east, and on the west the region of the Isthmus 

 of Panama, where the interchange of forms has continued to the present 

 time. That the middle American elements are far younger in South 

 America than the Archhelenic, is shown by their close relationships to 

 Antillean and Mexican forms, very few special genera having evolved. In 



FIG. 30. 



Distribution of the Oleacinidce. The dotted area stands for the genus Euglandina only, which 

 also occupies Mexico and Central America. Vertical shading in Europe for Tertiary, black for 

 recent species of Poiretia. 



most of the families only one or very few genera have invaded South 

 America out of a large number in the parent lands. Moreover, it is notable 

 that they have not extended far south in the east, where the Amazon 

 valley has proved a barrier to land-snails. Their distribution has been 

 along the Andes, spreading eastward in Bolivia and southern Brazil. 



Figures 30 to 34 illustrate the distribution of middle American groups 

 of land-snails incursive in South America. 



