2 EIGENMANN— FISHES OF SOUTH AMERICA. 



strictly fresh-water genera.- What is the origin of this fauna? 

 The solution to this problem is given by the distribution and relation- 

 ship of the io8 genera. We find : 



1. Sixty-five of the genera are also found east of the Andes 

 where they are for the most part widely distributed. They are 

 marked with the — in the second column of the table below. The 

 ancestors of the species of these 65 genera (66 if we include Rivulus, 

 which is in part marine) had a common origin with the species 

 foiuid in the Atlantic slope rivers. They constitute about 60 per 

 cent, of the total. 



2. Twenty-eight of the genera are modifications of some of the 

 above 65 or of other genera widely distributed east of the Andes. 

 The derivation is in many cases quite evident and direct. For in- 

 stance : 



XUipJiins is a modified Bunoceplialiis ; Cctopsorhamdla and Nan- 

 norhaindia are modified Rhamdia; Ercmophilns is a modified Pyg'i- 

 dium; Cheiridodus is a modified Plecostomus; Lebiasina is a modi- 

 fied Piahuc'ma and a per cent, of individuals of Lebiasina still revert 

 to Piabucina; Compsura and Pseudocheirodon are modified Cheiro- 

 don; Orthonophancs is a modified Brycon; Argopleura, Microgenes, 

 and Plicuacobrycon are modifications of Bryconamericus ; Landonia 

 is a modified Astyanax; Accstrorhynchns is a modified Accstro- 

 ccphalus; Ctcnolucinus is a modified Xiphostoma; and so on. In a 

 number of other cases the immediate origin is not so evident : Para- 

 stremnia and Rhoadsia form a distinct subfamily. Their young are 

 in all technical respects members of the Cheirodontinae, from 

 which they no doubt evolved. Gilbcrtolus is allied to the Characinxe; 

 Genycharax to Astyanax or Charax ; Gritndulus and Phanagoniatcs 

 are of the CJieirodontincc, Pterobrycon and Microbrycon of the 

 Glandidicaudincu . All are marked A in the first column. They con- 

 stitute nearly 26 per cent, of the total. 



Ten of the genera have either come from Central America or 



2 Exclusive of the marine or brackish water genera, Pristis, Hexa- 

 nematichthys, Sardinella, Stolcpliorus, Anchovia, Tylostirus, Mugil, Quert- 

 mana, Agonostomits. Joturus, Centropomus, Pomadasys, Tarpon, Dormita- 

 tor, Eleotris, Philypiius, Guavina, Gobiiis, Gohioncllns, Awaous, Gobioides, 

 Thalassophrync, Batrachoidcs, Cithariclitliys, Achirus. 



