The opah is a rare fish, swimming slowly near the surface and ranging very widely in all the warm seas. It was first noticed in Norway by Gunner, the good bishop of Throndhjem, about 1780. It was soon after recorded from Elsinore, Torbay, and Madeira, and is occasionally taken in various places in Europe. It is also recorded from Newfoundland, Sable Island, Cuba, Monterey, San Pedro Point (near San Francisco), Santa Catalina, Honolulu, and Japan.
The specimen studied by the writer came ashore at Monterey in an injured condition, having been worsted in a struggle with some better-armed fish.
Allied to Lampris is the imposing extinct species known as Semiophorus velifer from the Eocene of Monte Bolca near Verona, the type of the extinct family of Semiophoridæ. This is a deep compressed fish, with very high spinous dorsal and very long, many-rayed ventrals. Other related species are known also from the Eocene. There is no evidence of any close relation between these fishes with Caranx or Platax, with which Woodward associates Semiophorus.
The Semiophoridæ differ from the Lamprididæ chiefly in the development of the spinous dorsal fin, which is composed of many slender rays.
Suborder Zeoidea.—Not far from the Selenichthyes and the Berycoidei we may place the singular group of John Dories, or zeoid fishes. These have the ventral fins thoracic and many-rayed, the dorsal fin provided with spines, and the post-temporal, as in the Chætodontidæ, fused with the skull. Dr. Boulenger calls attention to the close relation of these fishes to the flounders, and suggests the possible derivation of both from a synthetic type, the Amphistiidæ, found in the European Eocene. The Amphistiidæ, Zeidæ, and flounders are united by him to form the group or suborder Zeorhombi, characterized by the thoracic ventrals, which have the rays not I, 5 in number, by the progressive degeneration of the fin-spines and the progressive twisting of the cranium, bringing the two eyes to the same side of the head. It is not certain that the flounders are really derived from Zeus-like fishes, but no other guess as to their origin has more elements of probability.
Fig. 195.—Semiophorus velifer Volta. Eocene. (After Agassiz, per Zittel.)
We may, however, regard the Zeoidea on the one hand and the Heterosomata on the other as distinct suborders. This is certain, that the flounders are descended from spiny-rayed forms and that they have no affinities with the codfishes.
Fig. 196.—Amphistium paradoxum Agassiz. Upper Eocene. (Supposed ancestor of the flounders). (After Boulenger.)