vi. 3 PALAEOZOIC ELASMOBRANCHS 179 



however, that the cladoselachians are very far from the earliest known 

 fishes and that in both ostracoderms (p. 125) and placoderms (p. 186) 

 fins are known that have a narrowly constricted base. We cannot 

 yet say for certain what has been the course of evolution of the paired 

 fins, but the fin-fold theory has much plausibility, in spite of the 

 difficulties raised by palaeontologists. 



Fig. 113. Pectoral fins of various fishes. 

 a, *Cladoselache\ b, *Pleur acanthus; c, Ncoccratodus; d, Gadits. (From Norman.) 



The cladoselachians represent the ancestral Devonian sharks, from 

 which all later forms have been derived. Animals of similar type 

 were fairly common in late Devonian and Carboniferous seas. The 

 ctenacanths, such as *Goodrichia, reached a length of 8 ft. Later 

 radiation of the selachians took place along three different lines, 

 represented by the three remaining orders shown in the classification. 

 The pleuracanthodians (*Pleuracanthus) were a specialized group of 

 freshwater carnivores. The tail was straight (diphycercal) and the 

 paired fins had become modified accordingly (see p. 137). The axis 

 was completelv freed from the body wall to give a paddle-like fin, 

 with pre- and post-axial rays, a type known as archipterygial (Fig. 

 113), because it was once supposed to be ancestral to all others. A 

 large spine on the head gives the group its name. Claspers were pre- 

 sent. These animals were common in the Carboniferous and Lower 



