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 '^ basal'' 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 



B 



^-^^9^^^^ 



Figs. 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 {Cladoselache) . 42. Plan of archaic unpaired fin in (larval) shark. 

 43. Unpaired fin of fossil Crossopterygian, Holoptychius. (After SMITH WOOD- 

 WARD.) 



A. Anal fin. B. Cartilaginous basal (fin support). C. Caudal fin. D. Dermal 

 margin of fin. D' . Anterior and D" . Posterior dorsal fin. K. Cartilaginous radial 

 (fin support). T. 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- 



