INTRODUCTION. XXKV 
one of the latter occurs also in the Balsas. In the last-named river is found a species 
(NV. boucardi) which has so far only been obtained elsewhere in Nuevo Leon, possibly 
a case of discontinuous distribution. 
The other Mexican Cyprinide 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 Siturip&, 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 Siluride of the Australian Region belonging to the marine 
Galeichthys, Plotosus, &c., 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 Galeichthys are found in the Eocene 
of Europe and North America. 
As in the case of the Characinide, 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 
(Gymnotide) is derived from the Characinide, the other two (Loricariide and 
Aspredinide) from the Siluride. 
In Mexico and Central America there are about sixty species, nearly half of which 
are marine Cat-fishes of the genera Arius, Galeichthys, and Atlurichthys. The fresh- 
water Silurids belong either to the widely distributed South-American genera Rhamdia, 
Pimelodus, and Conorhynchichthys, or to the North-American Amiuwrus and Leptops. 
Of these Pimelodus is represented only by two or three species in Panama, and 
Conorhynchichthys by one from the Usumacinta. Rhamdia, 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 widely distributed, 
ranging from the Great Lakes to Tamaulipas; a fourth is found in Texas and North- 
BIOL. CENTR.-AMER., Pisces, February 1908. d 
