38 THE AMERICAN CHARACIDAE. 



THE AMERICAN TETRAGONOPTERINAE. 



Whether we regard the Heterognaths as a single family, or as several fami- 

 lies, the fact remains that there are few groups of fishes within which the lines 

 of evolution are so clearly portrayed by existing forms as in the Characidae. 

 In order more satisfactorily to discuss their evolution I have divided the 

 family into a large number of small groups of genera with undoubted affinity. 



Several facts, aside from the general structure point to the Tetragonop- 

 terinae or perhaps the closely allied Cheirodontinae as the group nearest 

 to the ancestral Characins. 



1 . Tetragonopterid fishes have been found fossil in the Tertiary fresh-water 

 deposits at Taubate. 



2. They are found both in Africa and South America, the most nearly 

 allied genera on the two sides, Astyanax and Petersius, are scarcely generi- 

 cally distinct. No other subfamily has representatives on both sides of the 

 Atlantic. 



3. They are the dominant groups both in Africa, where they form more 

 than 36% of the Characins, and in America, where they form about 40%. 



4. They are found over the entire area of distribution both in America 

 and Africa. In America members of this subfamily form everywhere the 

 vanguard in the distribution. 



5. There are several lines of evolution diverging from the two subfamilies, 

 Cheirodontinae and Tetragonopterinae. 



Some of the lines of evolution radiating from different sections of the Tetra- 

 gonopterinae, or the closely allied Cheirodontinae are minor lines that have 

 not diverged greatly. For instance Diapoma and Stevardia are Tetragonop- 

 terids with modified opercles. The Gymnocharacinae are naked Tetragonop- 

 terinae. The Crenuchinae are apparently an offshoot from the Cheirodontinae. 

 The Stethaprioninae have developed a predorsal spine but are otherwise very 

 close to Tetragonopterus and especially to Moenkhausia. The Mylinae and 

 Serrasalmoninae have possibly diverged from the Stethaprioninae, increasing 

 the number of the dorsal rays, increasing the depth, compressing the ventral 

 surface, and adding spines and emphasizing the dentition without much alter- 

 ing it. Another offshoot from the Cheirodontinae has given rise to a series of 

 fishes with a decrease in the size of the mouth and the effectiveness of the 



