CLADOSELACHII 



187 



ring, are the only clearly specialised characters they possess ; and 

 in these they approach the Acanthodii. Another point of resem- 

 blance is the manner in which the paired fins merge posteriorly 

 into the body-wall. Important differences, however, separate these 

 two sub-classes. 



Family CLADOSELACHIDAE. Cladoselache, Dean ; Cleveland Shale 

 (Upper Devonian or Lower Carboniferous). 



Sub-Class 4. ACANTHODII. 



Among the earliest known fossil Vertebrata are certain spiny 

 fish, which are generally associated with the Elasmobranchii. 



B 



FK;. 1 "!'>. 



A, restoration otDtpUuxinihvistriahu, Ag. (after Traquair). B, scaling of Acanthoiles gracilia, 

 Beyr., enlarged (after Zittel). C, scales of the lateral line otProtaeanthodet pin nut us, Fr., enlarged 

 (after Kritsch). c.f, anal, c.f, caudal, d.f, dorsal, p.f, pectoral, and pv, pelvic fin ; 1 .1, lateral-line 



scales ; .j, spine. 



These Acanthodians extend from the Upper Silurian to the 

 Permian epoch ; their structure has been studied of late more 

 especially by Fritsch [139], Reis [351-2], A. S. Woodward [505], 

 Traquair [462, 460], Dean [111], and others. Shark-like in general 

 shape, they have a fusiform body, blunt snout, and distinctly 

 heterocercal tail. The hypochordal lobe of the caudal fin is large, 

 the epichordal scarcely developed. 



An anal and one or two dorsal fins are present, each with a 

 powerful spine at its anterior edge ; these spines are single 

 structures of vasodentine, as in the Holocephali, and probably are 

 enlarged denticles (Fig. 1 58). The web of the fin is attached in front 

 to the hinder edge of the spine, and along its whole extended base to 



