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Indiana University Studies 



C. Those finding their furthest north in the Atrato, some of 

 which probably moved from the San Juan to the Atrato, are ffcmi- 

 cetopsis amphiloxus, Loricaria jubata, Curimatus lineopunctatus, 

 Brycon oligolepis, Bryconamericus ortlwlepis, Parastremma sa- 

 diana, Tylosurus fluviatilis. Some of these may have originated 

 in the Atrato and migrated southward. 



D. Those confined to the two are Ancistrus centrolepis, Lehia- 

 sina multimaculata, Argopleura chocoensis, Nematobrycon am- 

 philoxus, Gephyrocharax chocoensis, Priapichthys nigroventralis, 

 Geophagus pellegrini, Acquidens biseriatus, Cichlasoma atro- 

 maculatum. The second of these probably migrated from the San 

 Juan to the Atrato ; most of the rest moved in the opposite direc- 

 tion. 



Those which find their furthest north in the Atrato, or their 

 furthest south in the San Juan, and which evidently moved 

 north or south, by no means indicate the limit of the intermi- 

 gration of species between the two river-basins. A glance at the 

 list of species will show about six genera which are represented by 

 distinct varieties or species in the two river basins and all of these 

 migrated from the one to the other in more remote times, or 

 what, in a measure, amounts to the same thing, they derived their 

 now distinct varieties or species from a common center. Note 

 particularly the genus Nannorhamdia, Hernia ncistr us, Roeboides. 



It is well to bear in mind that these lists are not exhaustive. 

 Many more species will probably be found in one or the other or 

 both of these basins. But while the details will have to be modi- 

 fied, the general conclusion that the Atrato-San Juan valley has 

 been used as a highway in fish dispersal will not be shaken. 8 



Nevertheless, the relation of the San Juan fauna to that of the 

 Atrato is less intimate than that of the Atrato to that of the 

 Magdalena, or that of the Paraguay to that of the Amazon, more 

 intimate than that of the Magdalena to the Orinoco fauna. The 

 continental divide at Istmina has been an effective barrier against 

 the southward migration of a number of genera. 



The genera Plecostomus, Prochilodus, and Leporinus swarm 

 in the Magdalena and Atrato. They are also found in Guayas 

 but not in the San Juan, Dagua, or anywhere between the Atrato 

 and Guayas basins. 



Those species or genera which find their furthest north in the 



8 Unless we should conclude that the present distribution dates back to the 

 period before the Atrato-San Juan valleys, when the tributaries of the Atrato and 

 San Juan emptied into the ocean. 



