Gonorhynchidæ.—To the Isospondyli belongs the small primitive family of Gonorhynchidæ, elongate fishes with small mouth, feeble teeth, no air-bladder, small scales of peculiar structure covering the head, weak dentition, the dorsal fin small, and posterior without spines. The mesocoracoid is present as in ordinary Isospondyli. Gonorhynchus abbreviatus occurs in Japan, and Gonorhynchus gonorhynchus is found in Australia and about the Cape of Good Hope. Numerous fossil species occur. Charitosomus lineolatus and other species are found in the Cretaceous of Mount Lebanon and elsewhere. Species without teeth from the Oligocene of Europe and America are referred to the genus Notogoneus. Notogoneus osculus occurs in the Eocene fresh-water deposits at Green River, Wyoming. It bears a very strong resemblance in form to an ordinary sucker (Catostomus), for which reason it was once described by the name of Protocatostomus. The living Gonorhynchidæ are all strictly marine.
In the small family of Cromeriidæ the head and body are naked.
The Osteoglossidæ.—Still less closely related to the herring is the family of Osteoglossidæ, huge pike-like fishes of the tropical rivers, armed with hard bony scales formed of pieces like mosaic. The largest of all fresh-water fishes is Arapaima gigas of the Amazon region, which reaches a length of fifteen feet and a weight of 400 pounds. It has naturally considerable commercial importance, as have species of Osteoglossum, coarse river-fishes which occur in Brazil, Egypt, and the East Indies. Heterotis nilotica is a large fish of the Nile. In some or all of these the air-bladder is cellular or lung-like, like that of a Ganoid.
Allied to the Osteoglossidæ is Phareodus (Dapedoglossus), a group of large shad-like fossil fishes, with large scales of peculiar mosaic texture and with a bony casque on the head, found in fresh-water deposits of the Green River Eocene. In the perfect specimens of Phareodus (or Dapedoglossus) testis the first ray of the pectoral is much enlarged and serrated on its inner edge, a character which may separate these fishes as a family from the true Osteoglossidæ. It does not, however, appear in Cope's figures, none of his specimens having the pectorals perfect. In these fishes the teeth are very strong and sharp, the scales are very large and thin, looking like the scales of a parrot-fish, the long dorsal is opposite to the anal and similar to it, and the caudal is truncate. The end of the vertebral column is turned upward.
Other species are Phareodus acutus, known from the jaws; P. encaustus is known from a mass of thick scales with reticulate or mosaic-like surface, much as in Osteoglossum, and P. æquipennis from a small example, perhaps immature. Phareodus testis is frequently found well preserved in the shales at Fossil Station, to the northwestward of Green River. Whether all these species possess the peculiar structure of the scales, and whether all belong to one genus, is uncertain.
Fig. 45.—Phareodus testis (Cope). From a specimen 20 inches long collected at Fossil, Wyo., in the Museum of the Univ. of Wyoming. (Photograph by Prof. Wilbur C. Knight.)
In Eocene shales of England occurs Brychætus muelleri, a species closely related to Phareodus, but the scales smaller and without the characteristic reticulate or mosaic structure seen in Phareodus encaustus.
Fig. 46.—Deposits of Green River Shales, bearing Phareodus, at Fossil, Wyoming. (Photograph by Wilbur C. Knight.)