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._ Extinct. The regular lozenge-shaped pattern of the integuments of these fishes is described by Sir P. Egerton thus: “Each scale bears upon its inner anterior margin a thick solid bony rib, extending upwards beyond the margin of the scale, and sliced off obliquely, above and below, on opposite sides, for forming splices with the corresponding processes of the adjoining scales. These splices are so closely adjusted that, without a magnifying power or an accidental dislocation, they are not perceptible. When _in situ_, and seen internally, these continuous lines decussate with the true vertebral apophyses.” In some genera the “pleurolepidal” lines are confined to the anterior part of the side. FIRST FAMILY--PLEUROLEPIDÆ. _Homocercal. Body less high. Fins with fulcra._ _Pleurolepis_ and _Homoeolepis_ from the Lias. SECOND FAMILY--PYCNODONTIDÆ. _Homocercal. The neural arches and ribs are ossified; the roots of the ribs are but little expanded in the older genera, but enlarged in the tertiary forms, so as to simulate vertebræ. Paired fins not lobate. Obtuse teeth on the palate and the sides of the mandible; maxilla toothless; incisor-like teeth in the intermaxillary and front of the mandible. Fulcra absent in all the fins._ These fishes abound in Mesozoic and Tertiary formations. _Gyrodus_, _Mesturus_, _Microdon_, _Coelodus_, _Pycnodus_, _Mesodon_, are some of the genera distinguished by palæontologists. (See Fig. 102, p. 201.) SEVENTH SUB-ORDER--LEPIDOSTEOIDEI. _Scales ganoid, rhombic; fins generally with fulcra; paired fins not lobate. Præ- and inter-operculum developed; generally numerous branchiostegals, but no gular plate._ FIRST FAMILY--LEPIDOSTEIDÆ.