PLE URA CAN THUS 



83 



J3 





CO 







,CQ 





-3 



c 



to have been present, 

 and suggest strongly I /^WW o 



continuous fin-fold char- 

 acters. (V. p. 40.) 



Pleuracanthus (Fig. 

 90), the third of the S' 

 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 







T3 

 O 





C 

 O 



O 



Oi 



3 

 O 



(H 



2 



u 



s-.S 



> in 



^•a 

 . a 



H ^ 



■5; -r I- 

 . <; 2 



■S .^ o 



D.^ -a 



•a jz . 



6 .-2 



Q ^.• 

 ,• -^ 



S bo 



o t: 



o 



rt <u rt 



E § ^ 



£ S . 



° § . 



bO cr 



-2 . O 



c 







Cv 



5 P ^ ^ .| 





I 





0) 



bo 



— . m 



< 3 S 



--; ^ .H 



S a. 





