44 



PAIRED FINS 



formation of fin spine, as in Acanthodian * (Figs. 32, 51, 

 and p. 81). But the protrusion of the line of the basals 

 must have brought with it a new use in the economy of 

 fish motion. The plane of the fin could now be directed 

 upward or downward ; the fin would become a direct aid 

 in propulsion ; it would acquire a paddle-like function ; it 

 could also be extended sideways as a check to motion. 

 Under these circumstances it is not unnatural that the 

 region of the concrescence of the fin rays should now be 

 transferred from the fin's anterior to the more useful pos- 

 terior (now distal) margin, and that the fin rays, as well as 

 the line of basals, should acquire a more jointed structure, 

 suited to flexible motions. The course of the differentia- 

 tion of fin structures may be traced from this point on- 

 ward, as Wiedersheim has shown, by means of a series of 

 gradational stages : from the conditions of Fig. 49 we may 

 in the present figures pass to those of Fig. 52, thence to 

 those of Figs. 53 and 54. In the pectoral fin of a modern 

 shark (Fig. 52) the basal cartilages, B, may still be com- 

 pared with those in the older form (Fig. 49-5) ; their distal 

 element {B, at the right of the figure), however, protrudes 

 from the body wall and is becoming surrounded by clus- 

 tered radials, R ; the cartilaginous elements, it is here 

 noted, have been placed in competition with the dermal 

 elements, and have already yielded them over half of the 

 fin area. In the next stage of the evolution, as in the 

 pectoral fin of a Permian shark (Pleuracanthus, p. 83, Fig. 

 53), the line of the basals is seen to boldly protrude from 

 the body wall and to have become distinctly jointed ; the 

 radials have surrounded its distal end, and taken a position 



* This homology proposed by the writer has not been accepted by Smith 

 Woodward; the spine is unquestionably encased outwardly by dermal den- 

 ticles. 



