312 COMPARATIVE ZOOLOGY OF CHORDATES 



Later on during development the muscle-buds between the 

 positions of the pectoral and pelvic fins come to nothing. 



The most primitive paired fins known are probably those 

 of the fossil Cladoselache, in which they are triangular flaps 

 with the apex pointing outwards and the broad base attached 

 to the side wall of the body. The radials are more or less 

 parallel to one another and stick out at right angles to the side 

 of the body. It is important to notice that the radials are 

 scarcely concentrated at all at their base ; in fact the base is 

 the broadest part of the fin. In the body- wall there are some 

 basal cartilages with which the bases of the radials articulate. 



The next step in the evolution of the fins was probably 

 the concentration of the radials at the base of the fin. The 

 result of this was that the stalk attaching the fin to the side of 

 the body became narrow, and the fin became free to move in a 

 greater variety of manners. The arrangement of the radials 

 was now in the shape of a fan as in the Osteolepidoti, and the 

 fin itself was in the form of a blunt paddle. The centremost 

 radials formed what may be called the axis of the fin, but this 

 is not well marked in primitive forms in which the fin is short. 



By a lengthening of the axis a pointed laurel-leaf-shaped 

 fin is arrived at, like that of Ceratodus (the so-called archi- 

 pterygium). This type of fin is also present in the fossil 

 Pleuracanthus, where it would seem to have evolved indepen- 

 dently from that of Ceratodus. The skeleton of the paired 

 fins of the primitive fossil Dipnoi resembles that of the 

 Osteolepidoti. 



On the other hand, by a shortening of the axis and reduction 

 of the radials, the web of the fin comes to be supported mostly 

 by the dermal fin-rays, and this is the condition of the higher 

 bony fish. 



The pectoral and pelvic girdles must have arisen in accord- 

 ance with the need for a firm point of attachment of the fins 

 in the wall of the body. The radials at the base of the fin 

 have fused together and grown inwards, and in so doing they 

 may enclose in foramina the nerves supplying the fin. In 

 the pectoral girdle it is usual to find a dorsal scapular and 

 a ventral coracoid element ; the pelvic girdle is not so well 



