INTEODUCTION. xxv 



one of the latter occurs also in the Balsas. In the last-named river is found a species 

 (iV^. Ijoitcnrdi) which has so far only been obtained elsewhere in Nuevo Leon, possibly 

 a case of discontinuous distribution. 



The other Mexican Cyprinidas are the representatives in Sonora (5 species) or in 

 Mexico north of the Lerma System and east of the Sierra Madre (13 species) of Western 

 and Eastern North-American types respectively. 



The family Silurid^, with about 1000 species, is practically cosmopolitan in tropical 

 and temperate regions and is especially abundant in South America, Africa, India, and 

 the Malay Peninsula and Archipelago. The true fresh-water types do not appear to 

 cross Wallace's Line, the Siluridte of the Australian Region belonging to the marine 

 Galeiclbthys, Plotosus, Sec, or to genera which may be regarded as evolved from these. 

 A number of genera are common to the Indian and African Regions, but the Neotropical 

 fresh-water types are all generically distinct from, although in some cases closely allied 

 to, those of Africa. Genera related to the existing Galeiclithys are found in the Eocene 

 of Europe and North America. 



As in the case of the Characinida?, a larger number of genera and species and a 

 greater diversity is found in the Neotropical Region than in any other. This parallelism 

 extends to the fact that of three families peculiar to the Neotropical Region, one 

 (Gymnotidae) is derived from the Characinidfe, the other two (Loricariidse and 

 Aspredinidse) from the Siluridse. 



In Mexico and Central America there are about sixty species, nearly half of which 

 are marine Cat-fishes of the genera Aruis, GaleiclitJiys, and JEluriclithys. The fresh- 

 water Silurids belong either to the widely distributed South-American genera Rhamdia, 

 Pimelodus, and ConorliynchichtJiys, or to the North-American Amiurus and Leidofs. 

 Of these Twielodns is represented only by two or three species in Panama, and 

 Conorhynchichtliys by one from the Usumacinta. Bliamdia, however, extends 

 northward to Oaxaca and Southern Vera Cruz, and includes a considerable number 

 of Central-American species. 



Leptops comprises a single species from the United States east of the Rocky 

 Mountains, which is also found in the southern tributaries of the Rio Grande. 

 Amiurus includes a species in China and about twenty in America, eleven of which 

 are found in Mexico and Central America. Three of these are Avidely distributed, 

 ranging from the Great Lakes to Tamaulipas ; a fourth is found in Texas and North- 



BiOL. CENTR.-AMER., Pisces, February 1908. d 



