THE AMERICAN TETRAGONOPTERINAE. 39 



dentition. This series includes the Anastomatinae, Chilodinae, Prochilodinae, 

 Hemiodontinae, Elopomorphinae, and finally, the toothless Curimatinae. 



Another fine diverging from the Cheirodontinae has given rise to the Sal- 

 mininae, Characinae, Acestrorhamphinae, Cynodontinae, and ultimately the 

 Hydrocyninae. 



Another line of divergence from the Cheirodon or Tetragonopterus group 

 led through the Bryconinae, Iguanodectinae, Pyrrhulininae, Piabuscininae, 

 to the Lebiasininae. A side branch from this leads to the Chalcininae which 

 points the way to the flying Gasteropelicinae. For the above reasons it seems 

 best to begin the detailed examination of the species with the Tetragonopterinae. 



It may be left an open question whether the African and American genera 

 owe their similarity to convergence or to community of origin. They are for 

 the most part small or minute fishes ranging from 50-200 mm. in length. 

 Myletes in Africa reaches 460 mm. The Tetragonopterinae are closely related 

 to the Cheirodontinae, Diapominae, Glandulocaudinae, Stethaprioninae, Bry- 

 coninae and to other subfamilies. In shape they vary from the fusiform 

 Creatochanes to the deeply compressed Tetragonopterus. In the majority 

 of the genera the mouth is small. The lower jaw is heavy, and on account of 

 the obliquity of the mouth, when the mouth is opened it is thrown forward, 

 so as to project beyond the snout; when the mouth is closed the teeth of the 

 lower jaw usually fit in behind the innermost series of the premaxillary. In 

 some genera from Africa there is an inner pair of conical teeth in the lower jaw. 

 These appear in lieu of an inner series of teeth which in the American Brycon 

 are still present on the sides of the lower jaw. Conical teeth like these appear 

 in isolated (not closely related) genera of other subfamilies both in Africa and 

 America, and, since they cannot be genetically connected, appear to offer an 

 example of homoplastic development. The skull is smooth in cross-section or 

 slightly grooved in the smaller specimens; two fontanels are well developed in 

 all the genera but Brycinus of Africa. In this genus there is no frontal fontanel. 

 The bridge between the fontanels is either flush with the surface or sunk beneath 

 it; the occipital process varies directly with the depth of the species, and serves 

 to bridge the space between the skull and first interneurals, i. e. the space over 

 the coalesced vertebrae which lack interneurals. In the deeper species the 

 process is curved or bent upward and is long, reaching as much as one third of 

 the distance of its base from the dorsal. In the slenderest species it is short and 

 insignificant. It is always grooved to its tip, the groove leading to the parietal 

 fontanel. 



