PLE URA CANTHUS 



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to have been present, fflffe^ * 



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and suggest strongly 

 continuous fin-fold char- 

 acters. (V. p. 40.) 



Pleuracanthus (Fig. 

 90), the third of the 

 well-known Palaeozoic 

 sharks, is widely differ- 

 ent from the Acantho- 

 dian : it suggests a tran- 

 sitional form between 

 the generalized Cladose- 

 lachian, on the one hand, 

 and the Dipnoan on the 

 other ; or, more accu- 

 rately, it demonstrates 

 that the stems of shark 

 and lung-fish were at one 

 time drawn very closely 

 together. It has thus 

 far occurred only in the 

 Carbon and Permian, 

 but may reasonably be 

 expected in lower hori- 

 zons as a contemporary 

 of the earliest lung- 

 fishes. 



Pleuracanthus is in 

 many ways the most in- 

 teresting and suggestive 

 member of the shark 

 group ; for it destroys 

 many of our conventional ideas as to the general characters 



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