The species of "Trunk-Fishes" number about 20, and are referable to 3 genera: Aracana, Ostracion, Lactophrys; all belong to the tropical seas, living near the bottom in shallow water. The genus Ostracion is represented by one species in the Upper Eocene.
The rigid box in which these fishes are encased entails more use of the dorsal and anal fins for progression than is customary among fishes. According to Brown Goode, "the propelling force is exerted by the dorsal and anal fins, which have a half rotary, sculling motion, resembling that of a screw propeller; the caudal fin acts as a rudder, save when it is needed for unusually rapid swimming, when it is used as in other fishes; the chief function of the broad pectorals seems to be that of forming a current of water through the gills, thus aiding respiration, which would otherwise be difficult on account of the narrowness and inflexibility of the branchial apertures. When taken from the water, one of these fishes will live for two or three hours, all the time solemnly fanning its gills, and when restored to its native element seems none the worse for its experience, except that, on account of the air absorbed, it cannot at once sink to the bottom." "No group of tropical fishes," says the same author, "is so thoroughly worked out in the writings of the fathers of natural history as this one. Over 200 years ago every species of trunk-fish now taken from the Atlantic was known to and described by the naturalists, and it is a well-deserved tribute to their discrimination as zoologists to say that none of the many efforts which have since been made to subdivide their species have been at all successful."
Fig. 438.—Ostracion quadricornis. × ½.
Division II.—GYMNODONTES.
Supraclavicle oblique, sometimes nearly horizontal; lower three pectoral pterygials enlarged and immovably united to the coracoscapular cartilage; upper pterygial small, suturally united to the scapula. Anterior vertebrae with bifid divergent neural spines. Basis cranii simple; suture between dentary and articular evident. Pelvis absent.
The spinous dorsal and the ventral fins are constantly absent, the praemaxillaries are united to the maxillaries, and the teeth are coalescent, forming a beak; parapophyses are not developed, and epipleurals are absent.
Fam. 1. Tetrodontidae.—Beak with a median suture. Interoperculum a long rod, attached to inner face of praeoperculum, sometimes connected with operculum, never with suboperculum. Gills 3. First 4 or 5 praecaudal vertebrae with bifid neural spine and closed neural arch. Skin naked or with movable spines, rarely with bony plates; belly inflatable. Vertebrae 17 to 29.
The "Puffers" or "Globe-Fishes" comprise about 60 species, referable to 5 genera: Tetrodon, Ephippion, Tropidichthys, Xenopterus, Chonerhinus. They inhabit all the tropical and warm seas, a few species being confined to fresh water. Remains of Tetrodon have been found in Upper Eocene and later formations. They are remarkable for the manner in which they inflate themselves with air.[[762]] The flesh of most species is poisonous.
Fam. 2. Diodontidae.—Beak without median suture. Interoperculum rod-like, attached posteriorly to the rod-like anterior limb of the suboperculum. Gills 3. All the praecaudal vertebrae with bifid neural spines. Skin with movable spines; belly inflatable. Vertebrae 21 or 22.