Eigenmann: The Serrasalmin.e and Mylin.e. 235 



body, are very frequently developed in the Serrasalminae and IMylina?. 

 These are as characteristic as the spots on the young of thrushes 

 (Titrdidce). These spots begin to appear when the young have 

 reached a length of between one and two inches. They are probably 

 most prominent when the fish is between four and eight inches long, 

 and they become obscured or disappear entirely in old age. The 

 juvenile color is different from the adult color in some other ways. In 

 Mylosoma ocellatus, for instance, there are cross-bars and an ocellus 

 on the sides, and in Colosoma there are cross-bars without an ocellus. 

 As in other fishes, the obliteration of the juvenile markings is in part 

 due to the development with advancing age of more superficially 

 located pigment. 



The Serrasalminae and Mylina^ are for the most part lowland 

 fishes. So far as recorded they reach the maximum elevation in the 

 Rio das Velhas of the San Francisco basin. They are not found in 

 the upper Potaro river, the Iguassu above the falls, nor in the Mag- 

 dalena or elsewhere beyond the Andes, nor in the coastwise streams 

 between the Rio Itapicuru and Rio Grande do Sul. 



The Serrasalminae can readily be distinguished from the Mylinae 

 by the teeth. The former have a single series of teeth in each jaw. 

 The latter have two series of teeth in the premaxillary and frequently 

 a pair of teeth behind the front series of the lower jaw. 



The interneural, the upper end of which carries the predorsal spine, 

 lies between the sixth and seventh neural spines. The vertebrae, 

 counting on the radiographs as abdominal those from the first which 

 carries a neural spine to the last which carries ribs, and as caudal all 

 those behind these, number as follows: 



Pygopristis denticulalus 14 -f 20 



Serrasalmo rhombeus 14 -\- 20 



Alyleus ellipticus 16 + 19 



Catoprion menlo 14+22 



Metynnis macidalus 14 + 19 



This count does not take into consideration the coalesced vertebrae 

 immediately adjacent to the head. 



Subfamily SERRASALMIN.-E. 

 Compressed, deep; ventral surface with serrae; premaxillary and 

 mandible each with a single series of notched or lobate teeth; palate 

 sometimes with teeth; dorsal comparatively long; ventrals minute, 

 anal variously developed; a procumbent predorsal spine. 



