eigenmann: fresh water fishes. 303 



IV. The Mexican Region. 



North of the Rio Balsas, and therefore north of the transition zone 

 is the Rio Grande de Santiago. It is the largest river of the Pacific 

 slope within the tropics or between the Rio Colorado and Tierra del 

 Fuego. It is also by far the richest in species, the Rio Balsas com- 

 ing next in size and but little further south containing but 12 species as 

 compared with the 48 of the Rio Grande de Santiago. Many of the 

 species of this region [Chirostonia] are indirect derivatives from the sea. 

 The only fish in this, the largest Pacific slope river, that suggests South 

 America, is Cichlasoma beaiii, and this occurs only near the coast. Its 

 contact with North America is shown by such genera as Lanipefra, Ameiu- 

 nts, Moxostoma, Algansea, Notvopis and Hybopsis. 



This basin above the falls separating the coastal section from the Mexi- 

 can plateau, together with the Valley of Mexico and the headwaters of the 

 San Juan, a tributary of the Panuco, which rises in the Mexican plateau, 

 harbors a fauna that is distinct from both the fauna of North America 

 and that of South America or even southern Mexico. Of this Meek says : 

 "From the area which includes the valley of Mexico, the headwaters of the 

 San Juan del Rio and the Lerma basin, there are at present fifty-four spe- 

 cies of fishes known (inclusive of coastal plain species), only two of which, 

 Moxostoma austyinitui Bean and Aztecitla vittata (Girard) [to these should 

 be added Cichlasoma beani'] have been taken in any other river basin. 

 These fifty-four species belong to twenty-one genera, eight of which are 

 peculiar to this region." Three of the other genera are confined to the 

 neighborhood and have probably emigrated from it, six are northern 

 genera, of which only one extends farther south than this area. "The 

 Lerma river system is far from being thoroughly explored, but apparently 

 its fishes are quite as distinct and characteristic as if the fauna were insular." 



The relation of each species in the following list to the northern (N), 

 southern (S), or Marine (Me), faunas, whether peculiarly Mexican (M) or 

 belonging to the coastal plain (C), is indicated by the respective symbols. 

 N Lanipetra spadicea Bean. 

 N Ameiiiriis dngesi Bean. 

 N Moxostoma aiistyinimi Bean. 

 N Xysfrosiis popoche ]or<\2in & Snyder. 

 N Algaiisea tincella (Cuvier & Valenciennes). 

 N Algausea dngesi Bean. 



