The Heterognathi.—Of these the order of Heterognathi seems to be the most primitive, but in some ways the most highly developed, showing fewer traits of degeneration than any of the others. The presence of the adipose fin in this group and in the catfishes seems to indicate some sort of real affinity with the salmon-like forms, although there has been great change in other regards.
The order Heterognathi, or Characini (ἕτερος, different; γνάθος, jaw), contains those Ostariophysi which retain the mesocoracoid and are not eel-like, and which have the lower pharyngeals developed as in ordinary fishes. In most cases an adipose fin is present and there are strong teeth in the jaws. There are no pseudobranchiæ, and, as in the Cyprinidæ, usually but three branchiostegals. The Characidæ constitute the majority of the fresh-water fishes in those regions which have neither Cyprinidæ nor Salmonidæ. Nearly four hundred species are known from the rivers of South America and Africa. A single species, Tetragonopterus argentatus, extends its range northward to the Rio Grande in Texas. None are found in Asia, Europe, or, with this single exception, in the United States. Most of them are small fishes with deep bodies and very sharp, serrated, incisor-like teeth. Some are as innocuous as minnows, which they very much resemble, but others are extremely voracious and destructive in the highest degree. Of the caribe, belonging to the genus Serrasalmo, known by its serrated belly, Dr. Günther observes:
"Their voracity, fearlessness and number render them a perfect pest in many rivers of tropical America. In all the teeth are strong, short, sharp, sometimes lobed incisors, arranged in one or more series; by means of them they cut off a mouthful of flesh as with a pair of scissors; and any animal falling into the water where these fish abound is immediately attacked and cut to pieces in an incredibly short time. They assail persons entering the water, inflicting dangerous wounds before the victims are able to make their escape. In some localities it is scarcely possible to catch fishes with the hook and line, as the fish hooked is immediately attacked by the 'caribe' (as these fish are called), and torn to pieces before it can be withdrawn from the water. The caribes themselves are rarely hooked, as they snap the hook or cut the line. The smell of blood is said to attract at once thousands of these fishes to the spot."
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).
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.