254 OBIGIN OF LIFE IN AMERICA 



I suggested above (p. 161) that the fresh- water mussels 

 (Unionidae) probably effected their principal dispersal during 

 the Mesozoic Era, and that this circumstance might account 

 for the fact that we possess distinct proofs of a migration of 

 species from North to South America. The great genus Unio 

 has recently been subdivided by Dr. Simpson into numerous 

 genera. One group of Unio (Plagiola), ranging from Mexico 

 to the Mississippi drainage "basin, reappears southward in 

 Nicaragua, another (Lampsilis) is known from Guatemala 

 to Yucatan. Other groups of Unio are confined to South 

 America. Unio-Tetraplodon occurs in Ecuador, having spread 

 from there into the Amazon valley. Unio-Castalina lives in 

 southern Brazil, Unio-Castaliella in Surinam and so forth. 

 Finally Unio-Diplodon principally inhabits Chile, Argentina 

 and Patagonia, while it reappears right across the Pacific 

 in New Zealand and Australia. 



The range of these groups of Unio is apparently very com- 

 plex in South America. 'Nevertheless, I quite concur with Dr. 

 Simpson * in the belief that they all are the descendants of 

 certain members of the family Unionidae, which wandered 

 slowly from one river system into another, during the Triassic 

 or some later Mesozoic Period, from North America to South 

 America. To judge from the general distribution of the Unio- 

 nidae in South America, they entered that continent from the 

 west and only reached the eastern States subsequently. The 

 group Unio-Hyria, as Dr. von Ihering f tells us, is nothing 

 but a modified Unio, which has comparatively recently pene- 

 trated from Guiana into Brazil. The most surprising fact 

 which is so strongly brought out in that author's remarkable 

 researches is, that, while these Unionidae live in company with 

 other families of fresh-water mussels in eastern South 

 America, in Central America, Ecuador, Peru and Chile, that 

 is to say westward of the Andes, Unios alone occur. This con- 

 firms the opinion I expressed several times in previous chap- 

 ters, that the faunistic interchange between North and South 

 America took place between the western portions of the two 

 continents. 



* Simpson, C. J., "Synopsis of the Najades," p. 507. 



t Ihering, H. von, " Archhelenis und Archinotis," p. 122. 



