24 



PISCES. 



Turning to the paired fins, the few known facts of Palaeon- 

 tology suggest that they have originated in precisely the same 

 manner, first by the crowding and concrescence, secondly by 

 the shortening of endoskeletal supports which were originally 

 parallel bars. It may even be that the unknown factor causing 

 the dorsal fold to persist at two points operated likewise in 



FIG. 22. 



Pleuracanthus (Xenacanthus) decheni; pectoral fin, showing basal cartilages and 

 clustered dermal rays (actinotricbia), nat. size. L. Permian ; Bobemia. 

 Left border preaxial, right border postaxial. (After A. Fritsch.) 



preserving the lateral fold only at two points. The oldest 

 known paired fins clearly observable, are those of the Devonian 

 Elasmobranch Cladoselache ; and here the cartilages are parallel 

 bars, each only segmented once at the line of the body- wall, the 

 inner segments exhibiting no fusion in the pelvic fin (fig. 21 B), 



