﻿Eigenmann: Fishes of Panama 



11 



Viewing the composition, of the Chagres fauna from a little 

 different angle, we find that there are 38 genera of fishes in the 

 Chagres and that 28 of these are also found in the Atrato or 

 Magdalena. Two genera, Compsura and Pseudocheirodon, 

 found in all the Panama rivers have closely related genera in the 

 Atrato and Magdalena from which their ancestors no doubt came. 

 Six of the 28 genera — Gambusia, Priapichthys, Mollienisia, 

 Poeciliopsis, Joturus, and Neetroplus — came from the north, 

 altho the first 3 now extend into the Atrato-Magdalena. Menidia 

 is a marine genus with species in the fresh waters in numerous 

 places and came from the ocean. Four are lowland or brackish- 

 water genera of the Gobiidse, which are found largely in Panama. 



As far as the genera give any indication, the Chagres fauna is 

 composed of about 70 per cent derivatives from the south, about 

 16 per cent derivatives from the north. The rest of the fauna 

 is composed of derivatives from the ocean. 



Eighteen, or about 40 per cent of the Chagres species were 

 found in the small streams opposite the Chagres emptying into 

 the Pacific. Forty-three per cent are also found in the Chepo 

 basin, 27 per cent in the Tuyra, and 18 per cent in the Atrato. 7 

 The drop in percentage from the Chepo to the Tuyra is due to 

 the fact that some northern species stop at the Chepo and do not 

 extend into the Tuyra {Gambusia episcopi, Mollienisia sphenops, 

 Joturus pichardi) and to the fact that some Chagres-Chepo species 

 are replaced by other species of the same genera in the Tuyra 

 (Piabucina panamensis by festce, Hoplias microlepis by mala- 

 baricus, Priapichthys tridentiger by the variety cana). 



The Chagres is the meeting-place of three streams of migra- 

 tion, the largest from the south, a smaller one from the north, 

 and another from the ocean. 



The Chagres fauna having come largely from the south, a 

 supplementary question is, Did it arrive by land or by sea? 



The Chagres drains into the Caribbean Sea. The nearest 

 large rivers to the south draining into the Caribbean are the Mag- 

 dalena and the Atrato. The Magdalena basin is much the larger 

 of the two and contains a much more varied fauna than the 

 Atrato. The bulk of the Atrato fauna came from the Magdalena. 



If the region between Buenaventura and the canal should be 

 depressed by 400 feet, the Atrato and San Juan valleys would 

 be converted into a channel, and so would the Canal Zone. Large 



7 Most of the 18 per cent are included in the 27 per cent of the Tuyra and 43 per cent 

 Of the Chepo. 



