The obtusely or acutely lobate pectoral fins articulate with the pectoral girdle by a single basal endoskeletal element. Nostrils on the ventral surface of the snout. Two dorsal fins and an anal fin. Dermal bones of the ethmoid region often fused with one another and with the premaxillae in front and the frontals behind to form a continuous rostral shield. Infra-dentary bones may be present. A series of lateral jugular plates often present in addition to the pair of principal plates. The Osteolepida first make their appearance in the Old Red Sandstone and Devonian formations, where they become abundant. They are also well represented in the Carboniferous, but only one family survived to the Mesozoic period, finally becoming extinct in the Upper Cretaceous. The following are the more important families:—
Fig. 273.—Restoration of Osteolepis macrolepidota. Old Red Sandstone. (From Traquair.)
Fam. 1. Osteolepidae.—Scales rhombic and thickly enamelled. Pectoral and pelvic fins obtusely lobate. Tail heterocercal. Teeth simple, not complicated by surface infoldings except quite at the base. Genera:—Osteolepis (Fig. 273), Thursius, Diplopterus (Middle Old Red Sandstone, Scotland), Glyptopomus (Upper Old Red Sandstone, Scotland), Megalichthys (Carboniferous and Lower Permian of Europe and North America).
Fig. 274.—Skull of a Rhizodont (Rhizodopsis sauroides), Lower Carboniferous. A, lateral view; B, the dorsal surface; and C, ventral view. an, Angular; d, dentary; f, frontal; i.d, infra-dentary; j, principal jugular plates; l.j, lateral jugulars; m, mandible; m.j, median jugular; mx, maxilla; o, orbit; op, operculum; p, parietal; p.f, post-frontal; p.mx, premaxilla; p.op, preoperculum; so, suborbital; s.op, suboperculum; sq, squamosal; st, supra-temporal; x, x′, cheek plates. (After Traquair.)
Fam. 2. Rhizodontidae.—Scales cycloid and overlapping. Paired fins obtusely lobate. Tail heterocercal, sometimes apparently gephyrocercal. Teeth with the external enamelled layer of dentine infolded towards the axis in the form of radially arranged folds. In some genera ring-like vertebral centra have been recognised and also a preoperculum. Genera:—Rhizodus, Lower Carboniferous of Scotland and Northumberland; Tristichopterus[[562]] (Fig. 275), Old Red Sandstone of Scotland; Eusthenopteron[[563]] (Fig. 276), Upper Devonian of Scaumenac Bay, Canada; Gyroptychius, Old Red Sandstone, Scotland; Rhizodopsis[[564]] (Fig. 274), Carboniferous of England, Scotland, Silesia, and North America; Strepsodus, Carboniferous of Great Britain, Ireland, and North America.
Fig. 275.—Restoration of Tristichopterus alatus. Old Red Sandstone. cl, Clavicle. Remaining reference letters as in Fig. 274. (After Traquair.)
