EIGENMANN: THE PYGIDIID.E, A FAMILY OP SOUTH AMERICAN CATFISHES. 269 



Bertoni, Paraguay. Later he sent me two more specimens, all three having been 

 taken from the gills of a large characin, Piaractus brachypomus. 



Ribeiro, of the National Museum of Rio de Janeiro, caught another very similar 

 member of this subfamily, Paravandcllia, among the water-weeds of the stream near 

 San Luis de Caceres, in the Upper Paraguay basin. 



With fishes as rare as these and as small as these, the question arises whether 

 two species are really different, or whether the described differences are due to the 

 fact that one worker uses a hand lens, and the other a binocular dissecting micro- 

 scope with an arc spot-light. The result's of the two instruments are comparable 

 to the effects produced by an old-fashioned cannon and a modern forty-two centi- 

 meter howitzer. Branchioica and Paravandcllia may prove to be synonymous. 



Distribution (Plates XXXVI-XXXIX.) 



In considering the distribution of the fresh-water fishes of South America I 

 found, among other things (Proc. U. S. Nat. Mus., XIV, 1891, p. 18) "that genera 

 of many species usually have a wide distribution, and conversely, genera of wide 

 distribution usually have many species." With one exception the number of 

 species of any genus of the Pygidiidm varies directly with the greatness of the area 

 over which it is distributed. Some genera consist of but one species, and that 

 restricted to but one, or a few neighboring localities. As far as known, Eremophilus 

 is all but confined to the plateau of Bogota, Scleronema to the center of the Uru- 

 guay basin, Acanthopoma to a part of the Huallaga basin, Stegophilus to the Upper 

 San Francisco basin, Paravandcllia to the Upper Paraguay basin, Branchioica to 

 the Lower Paraguay basin. The genera with more than one species invariably have 

 a wider distribution. Homodicetus, with two species, is limited to the lower and 

 central La Plata basin, Henonemus, with four species, to the Amazon basin, Hatch- 

 eria, with six species, to the Andes of central and southern Argentina and Chile, 

 and Pygidium, with sixty-three species, is found in all the mountain streams from 

 the Tuyra in southern Panama to central Chile and central Argentina, in the moun- 

 tain streams from Rio Grande do Sul to the Rio Sao Francisco, and sparingly in the 

 lowlands of Guiana and Brazil. The only exception to the general rule is Och- 

 macanthus, with three species, ranging from Guiana to Paraguay. 



The Pygidiinw are mountain forms, and while they are found in lowlands near 

 the mountains, we find the optimum in the plains of Bogota and in Lake Titicaca. 

 They are sometimes the last species to succumb in the struggle with adverse con- 

 ditions found in high altitudes, and they range further south (to latitude 47° 30' ), 

 than any other tropical American fishes. 



