FISHES OF SOUTH AND MIDDLE AMERICA 521 



6. The Marine Character of the Titicacan Fauna. 

 The fish fauna of Lake Titicaca, about as large as Lake Erie, con- 

 sists of a catfish, Pygidium rivulatum, and several species of Orestias, 

 a Fundulus-like pcecilid. The former belongs to a genus of mountain 

 catfishes universally distributed in mountain streams of South America. 

 It has succeeded in crossing all sorts of barriers, and has undoubtedly 

 migrated into Lake Titicaca from the streams surrounding it. The 

 genus Orestias, on the other hand, is confined to Lake Titicaca and the 

 streams and lakes immediately surrounding it. The latter have doubt- 

 less received theirs from Lake Titicaca, which, on its part, could only 

 have received them at the time it was an arm of the sea, in which its 

 nearest relatives flourish. 



7. The Patagonian Fauna. 

 The interest in the Patagonian fresh-water fish fauna is entirely 

 out of proportion to its diversity and centers largely in its origin. 

 Only about twenty-five species of fishes are known to live or enter the 

 fresh waters south of the line joining Valparaiso and Bahia Blanca. 

 These few species fall, according to their origin, into four distinct 

 groups. 



1. Immigrants from the sea (a) are in the process of acclimatiza- 

 tion (species of Menidia and Atherinichthys), or (5) may be looked 

 upon as long established (species of Percichthys and Percilia). Mem- 

 bers of (a) are found in all the rivers; members of (b) are found in 

 the north chieflv, but reach the Santa Cruz. 



2. Immigrants from the fresh waters on the north : a verv small 

 overflow from the extremely rich fauna to the north and still retaining 

 their generic affinity with the northern forms. Here belong the 

 species of the genera C heir don and Astyanax, which are very widely 

 distributed in tropical South America and are not known to extend 

 much south of the Bio Negro. 



3. Autochthons or of doubtful origin. Here belongs the highly 



9. Astyanax bimaculatus L. A Characin found everywhere in Tropical South 

 America, from Panama to Buenos Ayres east of the Andes, close relatives existed in the early 

 tertiary. This photograph of a specimen from Trinidad.* 



