PAIRED FINS 



45 



along the outer half of the hinder margin of the fin stem ; 

 the dermal region of the fin, D, has notably increased. 

 Indeed, the fin area in the modern bony fishes (Fig 145, 

 PF) may become entirely dermal, and the basal supports 

 greatly reduced and metamorphosed. In a final type of 

 fin (Fig. 54) the line of the basals has become widely spe- 

 cialized, and the characters of the archipterygium have 

 been attained : the fin stem is long, tapering, jointed ; the 

 radials occur as clearly along the hinder as along the ante- 

 rior margin; and, as in Figs. 52, 53, dermal rays contrib- 

 ute largely to the fin area. This form of fin may be noted 

 as most closely approximating in function the limb type of 

 land-living vertebrates. 



It has recently been urged that the lateral fold origin of 

 the paired fins as thus described is not confirmed by devel- 

 opmental studies, — the especial ground for this belief 

 being that in sharks these fins appear, even in very early 

 stages, as paired lappet-like outgrowths, destitute of inter- 

 vening fin membrane. The perfected fin fold is therefore 

 claimed to represent nothing more than a specialization to 

 bottom-living, since this condition is known to maintain in 

 earlier stages and in more primitive metamerism in the 

 development of skates : and as skates (p. 93) are well known 

 to represent a comparatively recent offshoot from the stem 

 of the sharks, it is accordingly inferred that the chief proof 

 of the lateral fold doctrine is destroyed. 



Since these objections, however, were raised, the struct- 

 ural conditions of the ancient shark of Figs. 49 and 50 

 have been described, and may be looked upon as the 

 weightiest evidence of the origin of paired fins from lat- 

 eral folds. Nor does it seem to the present writer that 

 the early character of the fin-fold metamerism of skates 

 is to be looked upon as an unexpected condition. Their 



