FRESH-WATER MUSSELS. I 53 



Unio auratus, from Chile, closely allied to Unio rhuacoicus of Uruguay 



„ arancanus „ „ „ „ „ faba ., 



,, atratus „ „ „ „ ,, lepidior „ 



„ montanus „ ,, „ „ „ Beskeanus of S. Paulo. 



They are so nearly allied that the question may be raised whether 

 they belong to one species or not. 



Of Crustacea, Parastacus and uEglcea Icevis are common to Chile 

 and Rio Grande do Sul, the latter having in both places the parasite 

 Temnocephala. Most important is the relation between the La Plata 

 and Chile fauna. The upheaval of the Andes in the beginning of the 

 tertiary period divided a formerly united territory into two parts, 

 between which no more exchange was possible. The genera and 

 species common to both parts represent the original stock of fresh- 

 water animals, but what is only found in Eastern South America 

 represents the tertiary addition, coming from the East. 



All we know about fossil fresh-water mollusca confirms this 

 hypothesis. The only genus of the Najadcn reaching far in the 

 Secondary epoch is the genus Unio, and this is the most widely 

 distributed, the only one really cosmopolitan. As in Chile, no 

 Anodonta have been found in Australia and New Zealand, nor 

 Ampvllaria either, though this genus is nowhere missing from the 

 Philippine Islands to Brazil. Dr. GUnther unites the fresh-water 

 fishes of Chile and New Zealand in one region ; the study of the 

 Najadce confirms it. Unio mutabilis, Lea, found in New Zealand and 

 Australia, has its nearest ally in Unio auratus of Chile. 



The absence of alligators and turtles from Western South 

 America can only be explained by their immigration having taken 

 place in tertiary times. As in Najadce there is also a very great 

 difference in the Chelonia of North and South America. A fauna 

 common to North and South America does not exist, but on the 

 contrary only the greatest contrasts. This fact can only be 

 understood by admitting a separation up to the end of the tertiary 

 epoch. 



The Iresh-water fishes of North America are those of the 

 paksarctic region ; but the Characinidce, Chromidce, &c, of South 

 America have their representatives in Africa, There must have been 

 land communication between Africa and South America. The 

 African Testudo sidcata is also found in Patagonia. 



Of the Pontaderice, common in South America, not only several 

 genera, but also one species, Eichhomia natans, occur in tropical 

 Africa, besides Pistia stratiotes, Lemna polyrltiza and other Brazilian 

 water-plants. The world-wide distribution of many species of water- 

 plants, from East Prussia to Australia, and from South America to 

 East India could not be understood if we did not suppose that they 

 are very old forms, existing already in the Secondary period, and 

 whose distribution occurred at a time when the now separated 

 continents were continuous. 



The marine littoral mollusca of East and West America consist, 

 with the exception of one Siphonaria and Cuspidaria patogonica, of 

 quite different species and partly of different genera. 



