162 THE AMEBIC AN NATURALIST [Vol. XLVI 



The Pceciliidas or killifishes, found in most warm portions of 

 the globe, comprise about two hundred very small species, the 

 largest Fundulus, AnaUeps, etc., seldom exceed 300 mm. In 

 the inviting streams of Central America where the majority of 

 species occur, adaptive radiation, as in every large family of 

 fishes, has taken place, from the central type Fundulus, result- 

 ing in the depressed catfish-like Rivulus, the beaked garpike-like 

 Belonesox, the sunfish-like Goodea, the carp-like Cyprinodon, etc. 



The origin of these small forms may probably be explained 

 by the selective migration and the successive adaptation of the 

 species occupying the deeper reaches of the streams. The 

 Pceciliidae usually feed at the surface and thus may tend to 

 disseminate throughout the full extent of the shallower waters. 

 Acanthophacelus has evidently been derived from Pcecilia, from 

 which it differs chiefly in the acquirement of two rows of retrorse 

 hooklets on the modified anal fin of the male, while Heterandria 

 is of a different type with conical, carnivorous dentition and 

 shortened alimentary tract. Further exploration may reveal 

 still smaller species of these interesting little fishes, which are 

 likely, however, to pass unobserved by all but the trained 

 naturalist. 



Arthur W. Henn 



Indiana University, 



