40 : LATERAL FOLD FINS 
supporting substance the fusion of the cartilages of the 
hindmost gill bars ; in its outward growth the median axis of 
the fin was first produced, the minor supports then arrang- 
ing themselves on both anterior and posterior margins. 
The fin of Fig. 52 was believed to represent a specially 
evolved (or “monoserial’’) form of the archipterygium: the © 
hindmost of its elements, 2, was homologized with the 
primitive fin stem, along whose posterior (post-axial) mar- 
gin the elements, 2, no longer occurred. The structures 
of Fig. 53 were adduced as a transitional stage in the dif- 
ferentiation of the biserial archipterygium (Fig. 54) into the 
monoserial form of Fig. 52. 
The theory of Gegenbaur as to the origin and evolution 
of the paired fins cannot be said to be in any way generally 
supported at the present time. The opposing view, that 
of their derivation from a continuous lateral dermal fin 
fold, based on the work of Thacher, Balfour, Mivart, — 
Dohrn, Wiedersheim, and others, is widely accepted, and 
continues to gain supporting evidence on the sides both 
of embryology and palzontology. 
In the following discussion of the paired fins the 
writer has mainly followed the recent studies of Wieders- 
heim.* 
The paired fins are believed to have arisen as balancing 
organs, accessory in function to the vertical fins. They 
probably occurred early in the line of descent as a response 
to a need for balancing the fish’s body, at the time when 
the vertical fin was separated into caudal, dorsal, and anal 
elements. There can be little doubt that they first arose 
in the line of the fish’s motion, and are known primitively 
(Figs. 49, 50), as a pair of keel-like lateral lappets arising 
somewhat ventrally, and directed outward and downward. 
* Das Gliedmassenshelet der Wirbelthiere, 1893. 
