XVIII 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 
erowth 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. 
On the other hand, their relationship to the Palaeoniscidae 
is indicated by the general disposition of the dermal bones of 
Fic, 287.—Restoration of the skeleton of Chrondrosteus acipenseroides. a.f, Anal fin ; 
c.h, cerato-hyal ; e, eye; h.a, haemal arches ; hym, hyomandibular ; 7, jugal ; 
n.d, neural arches; 7.c, notochord ; .s, neural spines; pe.f, pectoral fin; p.f, 
pelvic fin; s.o, suborbital; s.op, 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 G'yrosteus from 
the Upper Lias of Yorkshire. From an evolutionary point of 
view it 1s 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. Cat. 
Foss. Fishes, iii. 1895, p. 23. . 
