Two families of _Heterognathi_ are recognized: the _Erythrinidæ_, which lack the adipose fin, and the _Characidæ_, in which this fin is developed. The _Erythrinidæ_ are large pike-like fishes of the South American rivers, robust and tenacious of life, with large mouths armed with strong unequal teeth. The best-known species is the _Trahira_ (_Hoplias malabaricus_). [Illustration: FIG. 121.—_Brycon dentex_ Günther. Family _Characidæ_. Nicaragua. ] Among the _Characidæ_, _Serrasalmo_ has been already noticed. _Citharinus_ in Africa has very few teeth, and _Curimatus_ in South America none at all. _Nannocharax_ in Africa is composed of very diminutive fishes, _Hydrocyon_ exceedingly voracious ones, reaching a length of four feet, with savage teeth. Many of the species are allies of _Tetragonopterus_, small, silvery, bream-like fishes with flat bodies and serrated incisor teeth. Most of these are American. A related genus is _Brycon_, found in the streams about the Isthmus of Panama. Extinct _Characins_ are very rare. Two species from the Tertiary lignite of São Paulo, Brazil, have been referred to _Tetragonopterus_—_T. avus_ and _T. ligniticus_. =The Eventognathi.=—The _Eventognathi_ (ἔυ, well; ἔν, within; γνάθος, jaw) are characterized by the absence of teeth in the jaws and by the high degree of specialization of the lower pharyngeals, which are scythe-shaped and in typical forms are armed with a relatively small number of highly specialized teeth of peculiar shape and arranged in one, two, or three rows. In all the species the gill-openings are restricted to the sides; there is no adipose fin, and the broad, flat branchiostegals are but three in number. In all the species the scales, if present, are cycloid, and the ventral fins, of course, abdominal. The modification of the four anterior vertebræ and their connection with the air bladder are essentially as seen in the catfishes. The name _Plectospondyli_ is often used for this group (πλεκτός, interwoven; σπόνδυλος, vertebra), but that term originally included the _Characins_ as well. [Illustration: FIG. 122.—Pharyngeal bones and teeth of European Chub, _Leuciscus cephalus_ (Linnæus). (After Seelye.) ]