Fig. 286.—Restoration of Cheirodus granulosus. d.ect, Dermal lateral ethmoid; d.eth, dermal mesethmoid; d.sp, either a dermal sphenotic or a post-orbital bone; l.l, lateral line; orb, orbit. The pectoral fin is indicated in dotted outline. Other reference letters as in Fig. 284. (From Traquair.)
Fam. 3. Belonorhynchidae.—The systematic position of these Triassic forms is very doubtful, and it is by no means clear that they are Chondrostei at all.
Fam. 4. Catopteridae.—It is very probable that this widely-distributed Triassic family is an offshoot from the Palaeoniscidae. It agrees with the latter in the general character of the head and pectoral girdle and in the rhombic squamation, but differs from its progenitors and approaches the more modern Holostei in the semi-heterocercal condition of the tail, and in the approximate numerical agreement between the fin-rays and radialia of the dorsal and anal fins.[[581]]
Fam. 5. Chondrosteidae.—This family affords an interesting annectant link between the Palaeoniscidae and their degenerate living representatives the Polyodontidae and Acipenseridae. They agree with the latter in the general shape of the body, the growth of a preoral rostrum, and in the relatively small size of their ventrally-placed and probably protrusible mouth (Fig. 287). The skin is entirely scaleless, except on the upper lobe of the caudal fin, where, as in Polyodon and Acipenser, the primitive rhombic squamation and a series of fulcra are retained.
Fig. 287.—Restoration of the skeleton of Chondrosteus acipenseroides. a.f, Anal fin; c.h, cerato-hyal; e, eye; h.a, haemal arches; hym, hyomandibular; j, jugal; n.a, neural arches; n.c, notochord; n.s, neural spines; pc.f, pectoral fin; p.f, pelvic fin; s.o, suborbital; s.op, suboperculum; other reference letters as in Fig. 284. (After Smith Woodward.)
On the other hand, their relationship to the Palaeoniscidae is indicated by the general disposition of the dermal bones of the cranial roof, and the presence of a transverse row of supra-temporals and of an extensive series of branchiostegal rays (Fig. 288). The family is represented by Chondrosteus[[582]] from the Lower Lias of Dorset and Leicestershire, and Gyrosteus from the Upper Lias of Yorkshire. From an evolutionary point of view it is significant that the Chondrosteidae do not make their appearance until the Palaeoniscidae are approaching extinction.
The two remaining families, the Polyodontidae and the Acipenseridae, agree in presenting a remarkable leaven of characters otherwise distinctive of the typical Elasmobranch, associated with certain primitive features which they have doubtless inherited from some remote ancestral stock common both to existing Elasmobranchs and to the other primary groups of Fishes, and also with others obviously due to degeneration.
