INTEODUCTION. xix 



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 Goodea 

 have reached the upper tributaries of the neighbouring river-systems of the Balsas and 

 the Panuco. 



There can be no doubt tlrat tlie Lerraa System is tire original home of the 

 Characodontinse. 



The Poeciliinge start witli 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. Gamhusia 

 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. Poecilia and Girardinus comprise several species which differ 

 from Gamhusia 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 Pwcilia are Mollienisia, with one species from the Snuthern 

 United States and Tamaulipas and another from Lake Peten, and Xiijliophorus, with 

 five species in Atlantic coast streams from Mexico to British Honduras. 



The remarkable Anablejjs, 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 AnablejJS 

 appears to be Jenynsia from Argentina. 



The points to be noted in the distribution of the Cyprinodontida3 in Mexico and 

 Central America are the remarkable development of the Characodontinfe in the River 

 Lerma System, the presence of some endemic genera of the Poeciliin* in the Atlantic 

 coast rivers, the presence of only two genera {Eivulus and Anableps) which may be 

 regarded as probable immigrants from the south, and the absence of peculiar types iu 

 the Great Lakes of Nicaragua, which contain only two species, both widely distributed. 



The sub-order Ostaeiopiiysi, comprising the families Characinida), Gymnotidte, 

 Cyprinidn?, Siluridce, Aspredinidie, and Loricanida;, is an offshoot of the Malaco- 



