I52 JOURNAL OF SCIENCE. 



This fact has hitherto been much neglected, nor has it been 

 satisfactorily explained. Is it not surprising in the highest degree to 

 see on such a map Chile and Patagonia separated from the remaining 

 part of South America, and united with New Zealand ? We shall see 

 that the fresh-water mollusca show a similar appearance. And yet 

 the explanation is not difficult to be given. The study of the fresh- 

 water mollusca shows that the earliest palaontological genera are at 

 the same time cosmopolitan or most widely distributed in all parts of 

 the earth and on many of the larger islands, the genera Planorbis, 

 Physa, Limncea, and Ancylus are found. 



All these genera appear already in the Jura, partly even in the 

 the Carboniferous and represent the oldest, palrcozic fresh-water fauna. 

 The genus Unio shows very much the same distribution, appearing 

 also already in the Jura formation, whilst all the genera of the 

 Najadce, coming into existence much later, have a perfectly different 

 geographical distribution. Especially Anodonta, which appears only 

 in the tertiary period, and Ampullaria and its nearest allies show a 

 much narrower distribution, they are missing both in Chile and West 

 Peru as well as in New Zealand and Australia. When these genera 

 originated and began to spread, there was evidently no land-com- 

 munication between the Asiatic continent and its islands, and 

 Australasia. The numerous species of those genera could therefore 

 not reach Australia and New Zealand. 



A similar case lies before us in the fresh-water fauna of Chile 

 and Peru, which shows the old fresh-water genera, but not the 

 younger genera Ampullaria and Anodonta. 



Besides some widely distributed genera, the family of the Najadce 

 has a number of smaller genera, some of thftni inhabiting Africa, but 

 the greater number South America. Of the latter belong to the 

 Anodonta group : Aplodon, Spix (Monocondylcea), Mycetopus and 

 Columba (Leila) ; to the Unio group : Hyria, Castalia and Castalina. 



All the Najadce of South America just mentioned are nearly 

 related, as shown by the mode of hatching the embryos. The South 

 American embryos are developed in the internal branchiae, whilst the 

 European Najadce hatch their larva? in the external branchise. 

 Nothing is known of the development of the African Najadce. 

 Perhaps they will show some of the peculiarities of the South 

 American Najadce. 



The distribution of the genera of the Najadce over South America 

 is very peculiar. West of the Andes, in Peru and Chile, the genus 

 Unio alone is found, which occurs also everywhere in the eastern 

 parts. In the latter parts of South America we find in addition the 

 afore-mentioned genera, peculiar to this country. 



Previous to the upheaval of the Andes, in the place of the Chile 

 of to-day, there must have been land in existence richly provided with 

 fresh water ; which is proved by the genera of fresh-water animals 

 common to Chile and La Plata. The fauna of the La Plata and Rio 

 Grande do Sul waters is much richer than that of Chile, but the 

 genera of the latter are also found east of the Andes. Amongst the 

 Najadai is : — 



