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.