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 



FIQ.40 



B 



43 



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. R. 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- 



