5 1 4 PATAGONIAN EXPEDITIONS : ZOOLOGY. 



Fuego. The Unionidce and Mutelidce, groups copiously represented in the 

 La Plata, have their southern limit in the Rio Negro, each represented by 

 one species ; Planorbis has two, Ancylus one species, the rest of the fauna 

 being Patagonian. The Rio Negro fauna, including the region of Bahia 

 Blanca, is transitional also in land mollusks, the genera Bulimulus, Odon- 

 tostomus and Strophocheilus reaching their southern limit in the Sierra 

 Ventana, near Bahia Blanca. In the humid region west of the Cor- 

 dillera the faunal zones are deflected southward, the transition zone as 

 marked by the southern limit of Unionidcz being in the neighborhood of 

 Chiloe Island. That the transverse faunal zones of Patagonia run north- 

 ward in the Andean region is shown by the occurrence on the upper Rio 

 Chico of several Magellanic forms. 



I. LA PLATAN ZoNE. 1 



Includes the Plata and its tributaries in Argentina, Uruguay, Paraguay, 

 Brazil, and rivers flowing into the Atlantic in the Brazilian states of Sao 

 Paulo and Rio Grande do Sul. Except in Chilinidce, only the genera of 

 this fauna are mentioned below, the species being very numerous. 

 Chilinidae: Chilina fluminea, C.f. microdon, C. globosa, C. rushii, C. parva, 



C. portillensis, C. tehuelcha mendozana. 

 Physidae: Physa. 

 Lymnaeidae: Lymncea, Planorbis. 

 Ancylidae: Ancylus. 



Amnicolidae : Littoridina, Potamopyrgus, Potamolithus. 

 Ampullariidas : Ampullaria, Asolene. 

 Cyrenidae: Corbicula. 

 Sphaeriidae: Musculium, Pisidium. 

 Unionidae: Diplodon, Castalia, Castalina. 

 Mutelidae : Anodontites, Leila, etc. 



II. PATAGONIAN ZONE. 



A. Rio Negro Fauna? 



(All of the known aquatic mollusks of this fauna and those following 

 are enumerated.) 



1 Probably the La Platan fauna is a subdivision of the Amazonian, chiefly characterized by the 

 great development of Chilinidce and Amnicolidce. 



2 Including also the Rio Colorado, and known solely by the works of d'Orbignyand Doering. 



