B. Body encased in a carapace; no epipleurals; spinous dorsal, pelvis, and ventrals absent .......... 4. Ostraciontidae.
II. Gymnodontes.—Supraclavicle oblique or nearly horizontal; lower three pterygials enlarged and immovably united to the coraco-scapular cartilage; anterior vertebrae with bifid divergent neural spines; pelvis absent.
Beak with a median suture; interoperculum not connected with suboperculum; caudal fin present; body inflatable. .......... 1. Tetrodontidae.
Beak without median suture; interoperculum attached posteriorly to suboperculum; caudal fin present; body inflatable. .......... 2. Diodontidae.
Beak without median suture; interoperculum attached posteriorly to suboperculum; caudal fin absent, the body non-inflatable, truncate posteriorly, with the dorsal and anal fins confluent. .......... 3. Molidae.
Fig. 436.—Left side of pectoral arch of A, Triacanthus brevirostris, and B, Tetrodon mbu. cl, Clavicle; cor, coracoid; pcl, post-clavicle; p.r, pectoral rays; ptr, pterygials; sc, scapula; scl, supraclavicle.
Division I.—SCLERODERMI.
Supraclavicle vertical; pectoral pterygials not enlarged, movably attached by ligament to the scapula and coracoid, three to the former and one to the latter. All the vertebrae with the neural arches forming a single spine. Basis cranii more or less distinctly double; dentary and articular completely co-ossified.
Fam. 1. Triacanthidae.—Praemaxillaries protractile, free from the maxillaries; teeth in the jaws separate, conical or incisor-like; palatine arch firmly united to the skull. Gills 4. Praecaudal vertebrae with parapophyses; epipleurals present. Spinous dorsal fin with 2 to 6 spines. Ventral fins each represented by a strong spine, with an inner basal knob which locks it when everted, rarely with the addition of 1 or 2 rudimentary soft rays; pelvis present, firmly united to the pectoral arch. Scales small, sometimes spinous or bony. Vertebrae 20.