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Indiana University Studies 



to the naturalist, none of them are out of the ordinary evolution 

 of genera elsewhere in tropical South America. 



This brings us to the genera also found east of the Andes. A 

 few of these belong primarily to Venezuela and the Guianas. They 

 are: 



Creagrutus, found along the eastern base of the Andes from 

 the Rio Beni to Lake Valencia and even British Guiana. 



Gephyrocharax but recently discovered about Lake Valencia. 

 Its place of greatest abundance is western Colombia. 



Hemibrycon is found as far as Trinidad. 



Panaque and Hemicetopsis are also found in the Amazon. All 

 of the rest of the numerous genera enjoy a universal distribution 

 east of the Andes. 



It would scarcely be possible to isolate any place as large as 

 the Magdalena basin anywhere east of the Andes and north of the 

 La Plata that would not contain all of the rest of the genera. 



The question arises whether isolation by the formation of a 

 barrier is not the natural, most probable explanation of the 

 present Magdalena fauna. 



It is quite out of the question to transport all of these genera 

 over the present barrier formed by the Cordilleras of Bogota, 

 especially if we consider that the Cauca has not been able to con- 

 tribute anything to the Dagua or Patia over a divide much lower. 

 Either the Cordillera of Bogota is younger than the Magdalena 

 and its growth cut off the Magdalena area with its fauna from a 

 general lowland mass extending eastward from the Cordillera 

 Central or there has existed a possible route of migration perhaps 

 via Lake Maracaibo. The segregation could not have taken place 

 very recently, for in most cases the species are distinct on the two 

 sides of the Cordilleras. The segregation took place before the 

 lifetime of most of the present species. It is, nevertheless, start- 

 ling that about 20 per cent of the species of the Magdalena are 

 also found east of the Andes. 



The species found on both sides of the Cordilleras of Bogota 

 follow. Those marked with a star were taken between Bogota 

 and Barrigon. 



Pseudopimelodus zungaro (H.) 

 *Rhamdia sebse C. and V. 

 *Pinielodus clarias (Bl.) 

 *?Pimelodella chagresi (St.) 



Pseudoplatystoma fasciatum (L.) 



Sorubim lima (Bl. and Sch.) 



