PAIRED FINS 39 
* looked upon as including the elements of both the radials 
and the hzemal or neural processes and spines. 
The 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 chetropterygium, 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, or Archip- 
terygium. 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 
* SB. Gesell. f. Morph. Miinchen, 1894, p. 17. 
+ Gegenbaur, Das Flossenshelet der Crossopterygier. Cf. Morph. FB, 1894. 
