MEDIAN FINS 33 
tions of the fin lobe, and thence to taper in size toward 
the nodal points of the web. Each radial appears shortly 
to segment off a proximal joint, or “ dasa/” cartilage, B, to 
secure a more perfect attachment with the wall of the body. 
_The subsequent evolution of the fins appears to have 
been determined by two modifications of growth, —the 
clustering of the radial and basal elements, and the 
encroachment of newly formed marginal (distal) rays 
40-43.— Evolution of unpaired fins. 40. Plan of reduction of vertical fin 
web into its dorsal, anal, and caudal elements. 41. Arrangement of fin supports 
in primitive fin (C/adoselache). 42, Plan of archaic unpaired fin in (larval) shark. 
43- Unpaired fin of fossil Crossopterygian, Holoptychius. (After SMITH WoOoOD- 
WARD.) 
A, Anal fin. 2. Cartilaginous basal (fin support). C. Caudal fin. D. Dermal 
margin of fin. D’. Anterior and D”. Posterior dorsal fin. . Cartilaginous radial 
(fin support). 7. Actinotrichia, 
upon the functions of the older fin supports. Three 
stages in this metamorphosis will be seen in Figs. 41-43. 
- The first illustrates the dorsal fin of an ancient shark 
(Cladoselache, p. 79), and will at once be seen to present 
_ most primitive conditions: it closely resembles the theo- 
D 
