Morphology of the Fins 



ture practically identical with the archipterygium. Gegenbaur 

 suggests that the archipterygium actually has arisen in this 

 way in phylogeny." 



The fin-fold theory of Balfour, adopted by Dohrn, Weiders- 

 heim, Thacher, Mivart, Ryder, Dean, Boulenger, and others, and 



FIG. 55. Acanthoessus wardi (Egerton). Carboniferous. Family Acanthoessidce. 



(After Woodward.) 



now generally accepted by most morphologists as plausible, is 

 this: that "The paired limbs are persisting and exaggerated 

 portions of a fin-fold once continuous, which stretched along 

 each side of the body and to which they bear an exactly similar 



FIG. 56. Shoulder-girdle of Acan- 

 thoessus. (After Dean.) 



FIG. 57. Pectoral fin of Pleuracanthus. 

 (After Dean.) 



phylogenetic relation as do the separate dorsal and anal fins 

 to the once continuous median fin-fold." 



"This view, in its modern form, was based by Balfour on 

 his observation that in the embryos of certain Elasmobranchs 



