366 OKTMANN — DISTRIBUTION OF DECAPODS [Aprils, 



Eocene, Archiplata became united with the Chilian coast range by 

 the elevation of the Cordilleras, and it was thus connected with 

 Antarctica. And, further, in the beginning of the Tertiary, Archi- 

 plata connected, by way of Peru and Ecuador, with Central 

 America. This resulted in the final formation of South America 

 (in its rough outlines) which, however, was still in communication 

 with Antarctica. Finally, in the middle of the Tertiary, South 

 America was united with North America (in the isthmian region) 

 and was severed from Antarctica, and this represents the chief 

 features of the present conditions. 



We have seen that during the geological development of the 

 Americas interoceanic connections, which were directed east-westerly, 

 and united the waters of the Atlantic and Pacific Oceans, have 

 repeatedly played a part. These connections being extremely 

 important for marine zoogeography, have often been referred to by 

 various authors, but have generally been misunderstood, the value 

 of a determination of the exact time of their existence being 

 neglected. So it will be worth while here to put them together by 

 themselves. 



Interoceanic coit?iections of the Atlantic and Pacific Oceans. * 



i. In the Lower Cretaceous there were two connections : a. across 

 Mexico, and b. through the Orinoco valley. Both probably united 

 the marine fauna of the Mediterranean province with that of the 

 (Indo) Pacific. 



2. In the Upper Cretaceous we have the connection through the 

 Amazonas valley. This united the South Atlantic fauna, which, in 

 this period, formed part of the Indo Pacific, with the identical fauna 

 of the eastern Pacific. 



3. In the Older Tertiary there existed the Panamic connection, 

 which united the fauna of the Atlantic, the chief element of which 

 is Mediterranean, with that of the Indo-Pacific. 



4. In the Later Tertiary no interoceanic connection existed, the 

 Atlantic and Pacific faunas being sharply separated. These condi- 

 tions continued up to the present time. 



It is impossible to say at present whether there were any transi- 

 tions between these different stages. A coexistence and union of 

 the connections 1 and 2, at about the beginning of the Upper Cre- 

 taceous, is possible in the region of the Upper Orinoco and Upper 



