INTRODUCTION. X1X 
has extended northward to the rivers of Coahuila and Durango, and another has 
‘become marine and reached the coast of Lower California. Three species of (oodea 
have reached the upper tributaries of the neighbouring river-systems of the Balsas and 
the Panuco. 
There can be no doubt that the Lerma System is the original home of the 
Characodontine. 
The Peeciliine start with the genus Gambusia, which differs from Fundulus only in 
being viviparous and in having the anal fin of the male advanced and modified into an 
intromittent organ, the anterior rays being thickened and produced. Gambusia 
comprises about 14 species from the Southern United States, Mexico, Central 
America, Cuba, and Jamaica. The widely distributed G. gracilis (Atlantic streams 
from Illinois to Vera Cruz) enters brackish water; the others appear to be strictly 
fresh-water species. The Central-American Belonesox is a remarkable offshoot from 
the Gambusia type. Pecilia and Girardinus comprise several species which differ 
from Gambusia in the structure of the mouth and the dentition, which is modified in 
adaptation to their phytophagous habits. ‘These genera have a similar distribution to 
Gambusia, but in addition are well represented in the Lesser Antilles and in Brazil, 
a distribution no doubt to be accounted for by the fact that some of the species enter 
the sea. Allied to Pecilia are Mollienisia, with one species from tbe Southern 
United States and ‘Tamaulipas and another from Lake Peten, and Xiphophorus, with 
five species in Atlantic coast streams from Mexico to British Honduras. 
The remarkable Anableps, surface-swimming fishes with the projecting eyes divided 
horizontally into an upper part adapted for vision in the air and a lower for vision in 
the water, comprises three species—two from tropical South America, one from 
Southern Mexico and Central America. The males have the anal fin modified into a 
scaly conical intromittent organ with terminal orifice. The nearest ally of Anabdleps 
appears to be Jenynsia from Argentina, 
The points to be noted in the distribution of the Cyprinodontide in Mexico and 
Central America are the remarkable development of the Characodontine in the River 
Lerma System, the presence of some endemic genera of the Peeciliine in the Atlantic 
coast rivers, the presence of only two genera (Aivulus and Anableps) which may be 
regarded as probable immigrants from the south, and the absence of peculiar types in 
the Great Lakes of Nicaragua, which contain only two species, both widely distributed. 
The sub-order OstaRIOPHYsI, comprising the families Characinide, Gymnctide, 
Cyprinidxe, Siluride, Aspredinide, and Loricariide, is an offshoot of the Malaco- 
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