678 



pressed, but comparatively few in number — twenty-one — and 

 apparently lacking in intercalations. Were it not, accordingly, for 

 its acutely pointed shape this fin would correspond structurally to 

 the ventral fin of Fig. 4: it must in other words be looked upon 



Pig. 8. Pectoral of 

 spine-shaped fin of an- 



described Cladoselache 

 in the collection of Dr. 

 Clark. X »tout |. 



simple 

 present 

 of the 

 of 



en- 



as comparatively simple in structure, in general less modified, 

 perhaps, than any of the above mentioned forms of pectorals. And 

 it can be readily inferred that such a fin would have become 

 even more stout and spine-like if a greater number of rays, 

 or intercalated, were, as in C. kepleri or C. clarki, 

 in its construction. Among the notably Acanthodian features 

 Cladoselachian should be included the crust-like thickening 

 larged shagreen denticles along the anterior margin of the fin: for 

 it is evident that this incrusting by shagreen denticles may have borne 

 the same relation as ganoid plates to the stiffening of the fin rim : 

 and the subsequent fusion of their limy substance with the spine- 

 shaped cartilaginous mass of the compacted radials would, it seems 

 to the present writer, justly meet the criticism that the Acanthodian 

 spines are essentially dermal. 



In conclusion, the Cladoselachian must be regarded as aflfording 

 important evidence as to the dermal fold origin of the paired fins, — 

 not solely as presenting an actual type lateral-fold-like in character, 

 but as demonstrating that this type of fin was prevalent in very early 

 times. 



The present paper has emphasized the range in evolution of this 

 fin-fold fin in palaeozoic times, outlining the modifications which the 

 greatly extended lappet-shaped ventral fin of C. fyleri had under- 

 gone in the direction of producing the spine-shaped fins of the Acan- 

 thodians. The especial importance to the Thacher-Balfour theory 

 of the correlation of these highly differentiated sharks with the Clado- 

 selachians is obvious: for it would demonstrate that the strictly fin 

 fold type of paired fin was not merely present in the lowermost 



