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. a.f, Anal fin ; 

 c.A, cerato-liyal ; 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.) 



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. Cat. 

 Foss. Fishes, ill. 1895, p. 23. 



