PAIRED FINS 



39 



looked upon as including the elements of both the radials 

 and the haemal or neural processes and spines. 



TJie Paired Fins 



The paired fins of fishes claim an especial interest as 

 the precursors of the limbs of the land-living vertebrates. 

 In this light they have been widely studied, and many 

 schemes have been devised for the comparison of the parts 

 of the five-fingered extremity, or cJiciroptcrygiimi, of the 

 amphibian with the fin structures of many fishes. The un- 

 satisfactory character of these homologies, however, is felt 

 at the present time more generally than ever, and many 

 morphologists believe with Dr. Mollier* that the ancestral 

 form of the terrestrial limb cannot be found in any of the 

 known types of paired fins. 



Among fishes, on the other hand, there appears to be a 

 well-marked unity of plan in the varied forms of the 

 paired fins ; and there exists so perfect a gradation in 

 structural characters in the different forms that it seems 

 impossible to doubt their genetic kinship. Which fin, 

 however, must be looked upon as the ancestral type is still 

 disputed. Professor Gegenbaur has long maintained that 

 the fin of Fig. 54 (or, better, the pectoral fin of Fig. 147!) 

 is to be looked upon as the most primitive form, ox Arc hi p- 

 terygiuvi. It is a leaf-shaped fin, whose principal carti- 

 laginous supports are arranged in a row from base to tip 

 in the position of a mid-rib : and whose minor fin supports 

 are grouped more or less symmetrically on either side of 

 this axis (cf. Figs. 53, 54, 121, 123, 126). The archipteryg- 

 ium is believed by Gegenbaur to have had a centrifugal 

 origin : it arose behind the gill region, representing in its 



* .S"^. Gesell.f. Morph. Miinchen, 1894, p. 17. 



t Gegenbaur, Das Flossenskelet der Crossopterygier. Cf. Morph. JB, 1894. 



