THE ARCHIPLATA-ARCHHELENIS THEORY. 



I. Statement of the Theory. 



In volume IV, page 319, ^/ seq., of these Reports, Ortmann discusses 

 von Ihering's Archiplata-Archhelenis theory from the standpoint of marine 

 invertebrate fossils. Inasmuch as the evidence for this theory is mainly 

 derived from the fresh-water fauna, I propose to discuss the application 

 of the distribution of the fresh-water fishes of tropical America to this 

 theory. Before entering into the details I shall outline the theory in von 

 Ihering's own words,^ culled from his resume in Science (new series, XII, 

 857-864, 1900.) 



"The study of the fresh- water fauna, and especially of the Unionidae 

 of South America, gave me as a practical result the separation of the two 

 sub-regions 'Archiplata' and 'Archamazonia.' The first contains Chili, 

 Argentina, Uruguay and Southern Brazil, the second central and Northern 

 Brazil (Archibrazil) and Guyana, Venezuela, etc. (Archiguyana). Archi- 

 plata contains numerous genera of Mollusca, Crustacea, etc., that are com- 



* Ortmann has misread von Ihering in putting the line separating Archiplata and Archhelenis in 

 the Amazon valley. Ortmann says : 



" Archiamazonas or Archhelenis were separated completely from Archiplata by a broad stretch 

 of sea, which extended across the present continent, where is now the valley of the Amazon River, 

 the Cordilleras not being yet formed, and thus a broad communication existed between what is 

 now the Atlantic and Pacific Oceans." * * * 



Ortmann has been led into this mistake no doubt by the probable northward extension of 

 Archiplata along the present line of the Cordilleras to central Peru, where Archiplata most nearly 

 approached, not Archhelenis, but the Archipelago of the northern Cordilleras more nearly asso- 

 ciated with Archhelenis. 



" Communication was possible in a certain degree between the shores of Archiplata and Archi- 

 amazonas. This fact is most plainly seen in the presence of Navidad fossils in corresponding 

 deposits of northern Peru ; the Navidad beds were apparently deposited near the northwestern 

 extremity of Archiplata, and the Payta Tumbez beds near the southwestern extremity of Archi- 

 amazonas : at these places we have probably these two continents at their points of nearest ap- 

 proach to each other, and an exchange of marine forms may have been possible. The exchange, 

 however, was rendered more difficult by climatic conditions, and especially, although possible in 

 a certain degree between the Ecuadorian and the Chilian province, it was hard for the Caribbean 

 and Ecuadorian fauna to migrate further south, into the Patagonian region." Ortmann, 1. c. 



Ortmann includes the greater part of Archhelenis, i. c, Archibrazil, in Archiplata. 



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