EIGENMANN— FISHES OF SOUTH AMERICA. 3 



are modifications of immigrants from Central America, marked x. 

 They constitute 9.2 per cent, of the total. 



Five of the genera are modifications of immigrants from the 

 ocean, marked O. They constitute 4.6 per cent of the total. 



There is no genus in the entire area whose derivation is in 

 doubt. The fauna is largely a part of the general South American 

 fauna which has been pinched off by the formation of the Andes 

 and has gone its own way since the Andes have become high enough 

 to form an eft'ective barrier against the ready intermigration be- 

 tween the cisandean and transandean parts of the continent. 



The relation of the faunas of the different rivers in the area to 

 each other is receiving consideration in separate papers.^ 



Table of the Distribution of Genera. 



In the column " Origin " 

 A =: Genera which are evidently modifications of present-day Orinoco or 



Amazon genera. 

 C = Genera of the Pacific slope some of which are also in the Chagres, 



others in the Atrato. 

 X = Genera of Northern, Central American origin. 

 O = Genera of brackish water origin. 

 *:= Pacific slope genera found only in the Atrato or Chagres of the Atlantic 

 slope drainage. 

 The dash ( — ) indicates that the genus occurs in the particular river. 

 The addition mark (-|-) indicates that within the area the genus is limited 

 to the one river. 



Columns 4 to 7 in sequence give a line of migration, 11, 12, 13. 14, 15 

 give a different line, 11 being a duplicate of 4; columns 8, 9, 10 represent a 

 fauna distinct from that in the Magdalena-Atrato-San Juan system shown 

 in columns 3 to 7. The second column indicates the Maracaibo, Orinoco, or 

 Amazon basins. 



^ I regret to say, that so far, I have not been able to give consideration 

 to the Santa river nor to the lower courses of the Rio Loa and to those of 

 southern Peru. I hope that I may be able to visit these rivers in the near 

 future. 



