eigenmann: fresh water fishes. 



297 



1. Transition. 



2. Mexican. 



3. Brazilian. 



4. Andean.^ 



5. Patagonian. 



III. The Transition Region. 



While there are absolutely no North American intrusive elements in 

 South America, a number of Mississippi Valley forms have reached as far 

 as the Isthmus of Tehuantepec ; only a Lepisosteits, Carpiodes meridio- 

 nalis and possibly Aplodiuotus grnnnieiis extend farther. 



On the Atlantic slope the North American fauna gradually gives place 

 to or shades into the tropical fauna between the Rio Grande on the north 

 and the Isthmus of Tehuantepec on the south. The Rio Grande fauna is 

 distinctly northern, that of the Papaloapam on the south is as distinctly 

 tropical, although the former contains a few southern elements and the 

 latter a few northern ones. On the western slope, the southern element 

 makes its appearance farther south than on the eastern and is separated 

 from the northern realm by the Mexican fauna. The Mexican and Transi- 

 tion faunas have recently been made the subject of profound and exhaus- 

 tive study by Meek and I can not do better than to summarize his results. 



In this discussion the Petromyzonidcs, Bagriucc, Catostomidcs and 

 Cyprinidoi, being unquestionably derivatives from the north, are credited 

 to the north, although some of the genera are peculiar to the Mexican 

 or Transition res^ions. For similar reasons members of the CichlidcSy 

 CharacidcB, Pimelodince et al. are considered South American, although 

 some of the genera are peculiar to or dominant in Mexico and Central 

 America. Agonosto^nns and the Pocciliidce, which are dominant in Mexico, 

 Central America and the West Indies, are considered Transitional, although 

 they extend both into North America and South America. 



In the Rio Grande basin on the boundary of the United States there are 

 known to occur sixty-one species of fishes. Of this number three species, 

 Astyanax mexicamis, Cichlasoma pavonaceum and Herichthys cyano- 

 giittatum, are distinctly tropical, belonging as they do to Central or South 

 American genera ; forty-four are distinctly northern, and of the remainder, 

 two are tropical Gobies also found on the American Gulf coast, and eleven 



' The Titicacan basin probably should constitute a " region " distinct from the Andean north of 

 Titicaca. 



