PAIRED FINS 



43 



subsequent encroachment of the dermal fin margin. These 

 conditions may be briefly illustrated. The paired fins of a 

 primitive shark (Figs. 49, 50, v. p. 79) appear as the actual 

 lappet-shaped remnants of a continuous dermal fold. The 

 ventral fins (Fig. 50) have clearly retained even the out- 

 ward shape of the fin fold ; the supporting elements are 

 arranged in metameral order ; the radials, R, are unjointed, 

 extending from body wall to fin margin ; the basals, agree- 

 ing in number with the radials, are uniform in size, and as 

 yet unfused. The pectorals, acquiring more special func- 

 tions (Fig. 49), are enlarged in size, their basals, B, becom- 

 ing compressed and obscure. In these fins the effect of 

 concrescence is admirably marked ; the anterior fin margins, 

 pressed tail-ward in their plane of growth, become firm and 

 rigid, their elements stout and compact ; the basals, re- 

 sponding to this outward need, cluster more firmly together, 

 are compressed and fused, their anterior elements, largest 

 and stoutest, become inturned, their posterior elements, 

 slightest and most clearly metameral.* 



The next stage in the evolution of the paired fins is 

 clearly comparable to that already noted as occurring in 

 the dorsal fin of Holoptychius (Fig. 43), where the line of 

 basals, fusing compactly into a plate-like mass, had in- 

 turned its anterior, and protruded its posterior tip ; a 

 change apparently slight, but great in functional impor- 

 tance. Up to this stage the fin has been firmly implanted 

 in the body wall ; its motion, probably slight upward or 

 downward, served but to balance the fish, its fin rays, 

 tending to concentrate anteriorly, functioned as an efficient 

 cutwater. This process of concentration in the anterior 

 fin margin may have resulted, the writer believes, in the 



* The effect of the enlarged and clustering dermal denticles in strengthen- 

 ing the cutwater margin of the fin has already been noted (p. 28). 



