. The Cambridge natural history. Zoology. XVIIl CHONDROSTEI 489 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 sealeless, except on the upper lobe of the caudal fin, where, as in Polyodon and Acipenser, the primitive rhombic squamation and a series
. The Cambridge natural history. Zoology. XVIIl CHONDROSTEI 489 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 sealeless, 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. On the other hand, their relationship to the Palaeoniscidae is indicated by the general disposition of the dermal bones of. Fig. 287.—Restoration of the skeleton of Chrondrosteus acipenseroides. , Anal fin ; , cerato-liyal ; e, eye ; , haemal arches ; hym, hyomandibular ; j^ jugal ; , neural arches ; , notochord ; , neural spines ; , pectoral fin ; ^ pelvic fin; , suborbital; , suboperculum ; other reference letters as in Fig. 284. (After Smith Woodward.) 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 ^ 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 char- acters otherwise distinctive of the typical Elasmobranch, asso- ciated 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 1 Traquair, Geol. Mag. (3) iv. 1887, p. 248 ; Smith "Woodward, Brit. Mus.
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