Extinct. Tetragonolepis from the Lias (see Fig. [103], p. 207).
Fourth Family—Sphærodontidæ.
Body oblong, with rhombic ganoid scales; vertebræ ossified, but not completely closed; homocercal; fins with fulcra. Maxillary composed of a single piece; teeth in several series, obtuse; those on the palate globular. Dorsal and anal fins short. Branchiostegals.
Extinct. The type genus of this family is Lepidotus, so named from its large rhombic, dense, and polished scales. The dorsal is opposite to the anal, and all the fins are provided with a double row of fulcra. This genus ranges from the Lias to the Chalk; one species would seem to have survived into tertiary times, if it should not prove to be a Lepidosteus.
Fifth Family—Aspidorhynchidæ.
Body elongate, with ganoid scales; jaws prolonged into a beak; termination of the vertebral column homocercal. Fins with fulcra; a series of enlarged scales along the side of the body. Dorsal fin opposite to the anal.
Fig. 146.—Aspidorhynchus fisheri, from the Purbeck beds; m, mandible; a, presymphyseal bone.
Extinct; mesozoic. Aspidorhynchus has the upper jaw longer than the lower; very peculiar is the occurrence of a single, solid, conical bone, situated in front of the symphysis of the lower jaw, to which it is joined by a suture. Belonostomus with both jaws of equal length.