ACANTHODES 81 
may, perhaps, have been the cause of its extinction. 
The present writer believes 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 wardi, Egert. X about}. (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. 88); and on the roof of the head and 
mouth traces of membrane bones have appeared. Around 
Fig. 88.— Acanthodes gracilis, Beyr. Shagreen. x-aboutyts. (After ZITTEL.) 
a. Outer face. 4, 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. 
G 
