DEC. 28, 1916. FISHES OF PANAMA MEEK AND HILDEBRAND. 227 



well in the Chagres, as it is very rare there, nor has it undergone any 

 changes different from those of the Pacific slope. Ancistrus chagresi, 

 by whatever route it may have reached the Rio Chagres, is now abun- 

 dant there and may also be regarded as one of the early migrants. It 

 was found on the Pacific slope only in the, small Rio Chorrera, the 

 upper tributaries of which are opposite those of the Rio Trinidad. It, 

 therefore, seems probable that this species crossed from one stream into 

 the other before the gap between them became closed. The other 

 representative of this family, Plecostomus plecostomus, is of wide dis- 

 tribution and occurs on both slopes of Panama and southward. 



Among the characins evolution seems to be rather actively at work 

 in a number of species. Brycon chagrensis, Brycon petrosus, 

 Creagrutus notropoides and Rceboides guatemalensis all have closely 

 related opposites or relatives on the Pacific slope, but the influence of 

 the different environment during their period of separation has brought 

 forth certain definite and marked changes. Hyphessobrycon panamensis 

 alone of all the characins from the Rio Chagres has no near relative 

 elsewhere in Panama. The genus, however, reappears in Colombia. 

 The Panama representative was probably one of the early migrants 

 reaching the Chagres before the last passage between this river and the 

 Pacific streams was closed. If it actually reached the Atlantic slope 

 by that route, then we are obliged to conclude that the species was 

 unable to maintain itself on the Pacific slope and while it became extinct 

 there it thrived on the Atlantic. The rest of the species of the Chara- 

 cidcB known from the Chagres are common to both slopes of Panama. 

 Of these Astyanax ruberrimus at least also occurs throughout Colombia, 

 and Hoplias microlepis while replaced by H. malabaricus in the Rio 

 Tuyra Basin and in Colombia, oddly enough again reappears on the 

 western slope of Ecuador. 



The single representative of the family Gymnotidcs is of wide dis- 

 tribution, and does not appear to be undergoing any rapid changes. 



The representatives of the family Pceciliidoe are common to both 

 slopes, with the single exception of Gambusia nicaraguensis which can 

 scarcely be regarded a fresh water form, as it seldom occurs above 

 brackish water. 



Of the family Mugilidcs three species occur in the Rio Chagres. 

 Two of them at least are common to both slopes of Panama and Central 

 America as well as to the West Indies. 



Of the family Cichlida there are two species, Mquidens cceruleo- 

 punctatus and Geophagus crassilabris ,that are of rather wide distribution 

 and are found everywhere in the streams of Panama. Cichlasoma 

 maculicauda is found only on the Atlantic slope of Panama and north- 



