ACANTHODES 



8i 



may, perhaps, have been the cause of its extinction. 

 The present writer beUeves that Cladoselache may well 

 have represented the ancestral form of the Acanthodian. 

 The generalized structures of the former have given place 

 to a perfected dermal armouring, and a completed series 





Fig. 87. — Acanthodes jvardi, Egert. X about I. (Restoration slightly modified 

 after SMITH Woodward.) Coal Measures, England. 



of balancing fins. In Acanthodes the shagreen denticles 

 have thus become greatly enlarged and thickened, their 

 flattened and enamelled surfaces wedging closely to- 

 gether (Fig. ^^) ; and on the roof of the head and 

 mouth traces of membrane bones have appeared. Around 



Fig. 88. — Acanthodes gracilis, ^^yx. Shagreen. X about r^c. (After ZiTTEL.) 

 a. Outer face. b. Inner face. c. Isolated denticle. 



the eyes the many shagreen plates of Cladoselache have 

 fused into a group of four. Supporting the dermal gill 

 frills, there have also appeared rows of minute sculptured 

 plates (corresponding, perhaps, to those, BR, of Fig. 

 145), homologous, apparently, with shagreen denticles. 



