Gastrochisma.—Cleft of the mouth wide. Finlets behind the dorsal and anal fins. The ventral fins are exceedingly broad and long, and can be completely concealed in a fold of the abdomen.
G. melampus, from the coast of New Zealand; scarce.
Nomeus.—Cleft of the mouth narrow. No finlets. The ventral fin is long and broad, attached to the abdomen by a membrane, and can be received in a fissure of the abdomen.
N. gronovii is a common pelagic fish in the Atlantic and Indian Oceans; of small size.
Other genera belonging to this family are Psenes and Cubiceps.
Seventh Family—Scombridæ.
Body oblong, scarcely compressed, naked or covered with small scales; eye lateral. Dentition well developed. No bony stay for the præoperculum. Two dorsal fins; generally finlets. Ventrals thoracic, with one spine and five rays. More than ten abdominal, and more than fourteen caudal vertebræ.
The fishes of the “Mackerel” family are pelagic forms, abundant in all the seas of the tropical and temperate zones. They are one of the four families of fishes which are the most useful to man, the others being the Gadoids, Clupeoids, and Salmonoids. They are fishes of prey, and unceasingly active, their power of endurance in swimming being equal to the rapidity of their motions. Their muscles receive a greater supply of blood-vessels and nerves than in other fishes, and are of a red colour, and more like those of birds or mammals. This energy of muscular action causes the temperature of their blood to be several degrees higher than in other fishes. They wander about in shoals, spawn in the open sea, but periodically approach the shore, probably in the pursuit of other fishes on which they feed.[43]
Scombridæ are well represented in tertiary formations: in the Eocene schists of Glaris two extinct genera, Palimphyes and Isurus, have been discovered. In Eocene and Miocene formations Scomber, Thynnus, and Cybium are not uncommon.
Scomber.—The first dorsal continuous, with feeble spines; five or six finlets behind the dorsal and anal. Scales very small, and equally covering the whole body. Teeth small. Two short ridges on each side of the caudal fin.