51 8 The Class Elasmobranchii or Shark-like Fishes 



garding AcaiitJiocssits as an ally and perhaps descendant of 

 the primitive Cladoselache. Dr. Dean observes: 



"In the foregoing classification it will be noted that the 

 Acanthodia are regarded as included under the first order 

 of sharks, Pleuropterygii. To this arrangement Smith Wood- 

 ward has already objected that the spines of Acanthodians 

 cannot be regarded as the homologues of the radial elements 

 of the Cladoselachian fin (which by a process of concrescence 

 have become fused in its interior margin), since he believes 

 the structure to be entirely dermal in origin. His criticism, 

 however, does not seem to me to be well grounded, for, although 



Fig. 305. — Chmaiius scutiger Eserton, restored. Family Diplacanlhidce. 

 (After Powrie, per Zittel.) 



all will adm't that Acanthodian spines have become incrusted, 

 and deeply incrusted, with a purely dermal calcification, it 

 does not follow that the 'nterior of the spine has not had primi- 

 tively a non-dermal core. That the concrescence of the radial 

 supporting elements of the fin took place pari passu with the 

 development of a strengthening dennal support of the fin 

 margin was the view expressly formulated in my previous 

 paper on this subject. It is of interest in this connection to 

 recall that the earliest types of Acanthodian spines were the 

 widest, and those which, in spite of their incasing dermal cal- 

 cification, suggest most clearly the parallel elements represent- 

 ing the component radial supports. There should also be 

 recalled the many features in which the Acanthodians have 

 been shown to resemble Cladosclaclic." 



From these primitive extinct types of shark we may pro- 

 ceed to those forms which have representatives among livino- 

 fishes. From Cladosclaclie a fairly direct series extends throuoh 



