Scales ganoid, smooth like the surface of the skull. Two dorsal fins; paired fins obtusely lobate. Teeth conical. Caudal heterocercal.
Extinct. Diplopterus, Megalichthys, and Osteolepis from Devonian and Carboniferous formations.
Third Family—Coelacanthidæ.
Scales cycloid. Two dorsal fins, each supported by a single two-pronged interspinous bone; paired fins obtusely lobate. Air-bladder ossified; notochord persistent, diphycercal.
Extinct. Coelacanthus from carboniferous strata; several other genera, from the coal formations to the chalk, have been associated with it—Undina, Graphiurus, Macropoma, Holophagus, Hoplopygus, Rhizodus.
Fourth Family—Holoptychiidæ.
Scales cycloid or ganoid, sculptured. Two dorsal fins; pectorals narrow, acutely lobate; dentition dendrodont.
Extinct. In this family a peculiar type of dentition is found—the jaws are armed with two kinds of teeth, small ones serially arranged, and much larger fang-like teeth disposed at long intervals. Both kinds show at their base in transverse sections a labyrinthic complexity of structure, numerous fissures radiating from the central mass of vasodentine which fills up the pulp cavity, and sending off small ramifying branches. Genera belonging to this family are Holoptychius, Saurichthys, Glyptolepis, Dendrodus, Glyptolaemus, Glyptopomus, Tristichopterus, Gyroptychius, Strepsodus, from Devonian and Carboniferous strata.
SIXTH SUB-ORDER—PYCNODONTOIDEI.
Body compressed, high and short or oval, covered with rhombic scales arranged in decussating pleurolepidal lines. Notochord persistent. Paired fins without axial skeleton. Teeth on the palate and hinder part of the lower jaw molar-like. Branchiostegals, but no gular plates.