ACANTHOD1I 



191 



plate is added above ; while in the Diplacanthidae a girdle is com- 

 posed of two ventral claviculars and two larger dorsal supra- 

 claviculars, all of bone-like tissue (Fig. 163). The endoskeleton of 

 the fins must have been very much reduced or absent in most 

 cases, in correlation with the great development of the dermal 

 spines; but Acanthodes is said (Reis [352]) to possess some short 

 radials at the base of the pectoral fin. 



The Acanthodii were placed with the 'Ganoids' in the older 

 classifications; Huxley [227] considered them to be rather 

 intermediate between these and the Elasmobranchs ; A. S. 

 Woodward places them with the Elasmobranchs [505]. That 



FIG. 162. 



Aatitihodcs Wanli, 

 Eg. ; Upper Carbonil'., 

 Staffordshire. Skele- 

 ton of pectoral tin 

 and girdle, b, ' supra- 

 clavicular"; r, cerato- 

 trichia ; s, tin - spine. 

 (From A. S. Wood- 

 ward, Brit. Mus.Catal.) 



FIG. 163. 



Dipldcanthus strkitus, Ag. ; Lower Old 

 Red Sandstone, Scotland. Skeleton of 

 pectoral arch and tin. d, ' supraclavicular ' ; 

 id, ' clavicular ' plate ; ft, basal region of 

 pectoral fin ; s, anterior, and m, posterior 

 spine. (From A. S. Woodward, Brit. Mus. 

 C'atal.) 



FIG. 161. 



Pnrexus falontu-s, 

 Powrie ; Lower Old 

 Red Sandstone, Scot- 

 land. Skeleton of pec- 

 toral girdle and tin. 

 fc, 'supraclavicular' ; 

 c}, ' clavicular ' plate ; 

 s, tin-spine. (From 

 A. S. Woodward, 

 Brit. Mus. Catnl.) 



they are far removed from any known Osteichthyes is shown by 

 the absence of an operculum, of marginal jaw-bones, etc. (for it 

 must be remembered that the jaws of the Acanthodii represent 

 the palato-quadrate and Meckel's cartilage) ; affinity, on the other 

 hand, is shown in the structure of the scales, and the development 

 of hard tissue which bears some resemblance to bone. On the 

 whole, the Acanthodii seem to be nearer to the Chondrichthyes, 

 with which we shall place them on account of the general form of 

 the body, structure of the skeleton, the amphistylic character of 

 the jaws, the position of the lateral line, and the presence of 

 ceratotrichia. They resemble the Cladoselachii in general shape, 

 in the jaws, fins, and heterocercal tail, and in the circumorbital ring 

 of plates. It would be interesting to compare the finer structure 

 of the scales in these two groups. Dean [103] considers the Acan- 

 thodii to be related to, but more specialised than, the Cladoselachii. 



