Scales small or minute, sometimes hard and rough and firmly joined in vertical series; bony plates may be present along the base of the vertical fins. Air-bladder present.
Twelve species are known from the Atlantic and Pacific Oceans, referable to 5 genera: Grammicolepis, Cyttus, Cyttopsis, Zenion, and Zeus. Oreosoma was founded on a young form of a fish allied to Cyttus. Remains of Zeus occur in the Oligocene, and Cyttoides, from the same period, has been compared with Cyttus.
The well-known John Dory (Zeus faber) is much valued for the table.
Fam. 2. Amphistiidae.—The only known representative of this family, the Upper Eocene Amphistium paradoxum, originally described as a Pleuronectes, has much in common with the Zeidae, from which it differs in the smaller number of vertebrae (10 + 14), and in the dorsal and anal spines being more reduced, adnate and continuous with the series of soft rays; the scales are more normal and imbricate; ventral fins with 1 spine and 8 soft rays. This fish appears to realise in every respect the prototype of the Pleuronectidae before they had assumed the asymmetry which characterises them as a group.
Fig. 417.—Restoration of Amphistium paradoxum. × ½.
Fam. 3. Pleuronectidae.—Head asymmetrical, the skull twisted in front, with the two orbits on one side in the adult; the side of the body bearing the eyes and turned upwards in life being coloured, the other side colourless and blind. Mouth more or less protractile. Gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones usually separated, rarely imperfectly united. Vertebrae 24 in the most generalised form (Psettodes), varying from 28 to 65 in others, the praecaudals mostly with more or less developed transverse processes, which may be directed downwards and become converted into haemal arches; ribs and epipleurals present. Caudal fin, if well developed, supported by a large hypural usually without basal spine or knob. Dorsal and anal fins much elongate, without spines, the former often extending on the head. Paired fins often reduced, sometimes absent; if fully developed and normally formed, the bones of the pectoral and pelvic girdles as in the Zeidae. Ventral fins usually with 5 to 7 soft rays.
Scales usually imbricate, cycloid or ctenoid; rarely absent; bony tubercles sometimes present. Air-bladder absent.
Most species, and even genera, are either sinistral or dextral, but this is inconstant in some, including the most generalised genus, Psettodes. The very young are transparent and symmetrical, with an eye on each side, and swim in a vertical position like other Fishes. These larval forms have been described as distinct genera, under the names of Peloria, Bibronia, Charybdia, etc. As they grow, the eye of one side moves by degrees to the other side, where it becomes the upper eye. If at that age the dorsal fin does not extend to the frontal region, the migrating eye simply moves over the line of the profile, temporarily assuming the position which it preserves in Psettodes, Atheresthes, and Platysomatichthys; in other genera, the dorsal fin has already extended to the snout before the migration takes place, and the eye, passing between the frontal bone and the tissues supporting the fin, appears to pass from side to side through the head, as was believed by some of the earlier observers.[[728]]
Flat-fishes are a large group of some 500 species, mostly marine, a few species related to the Soles being confined to the fresh waters of South America and the Malay Archipelago. They range from the Arctic Circle to the southern coasts of the Southern Hemisphere; many occur at great depths (Citharichthys dinoceros down to 955 fathoms). Well-preserved remains referred to Psetta occur in the Upper Eocene, and a species of Solea is known from the Lower Miocene.