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, 2, may still be com- 
pared with those in the older form (Fig. 49 2) ; their distal 
element (2, 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 — x 
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. 
