82 



ACANTHODIAN SHARKS 



Further resemblances to Cladoselache are to be traced in 

 the position of the fins, gill slits, eyes, mouth, nasal cap- 

 sule, and in the structures of the caudal fin (Kner), and of 

 the lateral line. The teeth, however, are no longer of the 

 derm-denticle pattern ; they have become few in number, 

 large, and "degenerate" in their fibrous structure (Fig. 



Z^,A). The fins are clearly more per- 

 fect balancing organs than those of 

 the older shark ; their anterior rim is 

 formed by a stout spine, representing, 

 the present writer believes, the con- 

 crescence of the radial fin supports ; 

 it is heavily crusted over with the 

 calcifications of shagreen denticles. The functional fin 

 area has thus become dermal, and is lacking in supports, 

 excepting in the pectoral fin. This, as the most highly 

 specialized of all the body fins (p. 41), appears in some 

 cases to have evolved strengthening (dermal) rays in its 

 proximal portion (as in Figs. 87 and 32). 



Fig. 88 A. — Teeth of 

 Acatit/iodopsis wardi. X i. 

 From sketch after speci- 

 men in British Museum. 





/ 



Fig. 89. — Climatius scutiger, Egert. X i. (From ZiTTEL, after PoWRIE.) 

 Old Red Sandstone, Forfarshire. 





In connection with these fin structures the remarkable 

 Acanthodian, Climatms (Fig. 89), should finally be men- 

 tioned. In this form the paired fins are represented by a 

 series of fin spines whose size grades backward from the 

 pectoral region ; a series of paired fins appear, therefore, 



