DEVONIAN FISHES OF IOWA 99 



To the above description may be appended also the following 

 paragraph from another article by the same author :* 



"In conclusion, one word of protest against the American idea 

 that the paired fins of Cladoselache can be compared with those 

 of an Acanthodian.f We venture to maintain that these fins are 

 fundamentally different in every respect. In Cladoselache the 

 cartilages of the internal skeleton are well developed and sup- 

 port the whole fin membrane; in Acanthodians, whatever view 

 we may adopt as to the naming of the parts, these cartilages are 

 as much reduced as in a modern herring. Dr. Dean speaks of 

 the "radials" of Cladoselache as if, by fusion, they might readily 

 became a fin spine like that of the Acanthodian Parexus; but 

 the former are cartilage and endoskeletal, the latter is merely the 

 ordinary dentine and therefore presumably exoskeletal. The 

 problem of the primeval sharks continues to present endless 

 difficulties, but these are only multiplied by such comparisons. 

 In the present writer's opinion, the pectoral of Cladoselache is 

 more remotely connected with that of the Acanthodians than 

 is that of a modern Siluroid with the pectoral of the Devonic 

 Holoptychius. Everything still tends to show that the very 

 highest Elasmobranchs lived simultaneously with almost the 

 lowest in late Paleozoic times; while sharks and skates now- 

 adays are a comparatively degenerate race." 



We have already remarked that this genus furnishes impor- 

 tant support for the doctrine of the evolution of fins, which now 

 ranks as a fairly well demonstrated principle. In the first place, 

 it is important to bear in mind that the paired fins of Cladosel- 

 ache are the oldest known structures of their kind which are 

 clearly observable; and secondly, they approach more closely 

 than any others to the hypothetical primitive type from which 

 all paired limbs have been derived. Briefly stated, the lateral 

 fin-fold theory assumes that fishes originally possessed on each 

 side of the body a continuous fold of the integument, strength- 

 ened by parallel cartilaginous rods extending outwards from the 

 body wall, this fold becoming subdivided into the pairs of pec- 

 toral and pelvic fins as we know them in modern forms. 



^Woodward, A. S., The Problem of the Primeval Sharks. Natural Science, 

 1895, 6, p. 42. 



tThe Fin-fold Origin of the Paired Limbs, in the Light of the Ptychopterygia of 

 Palaeozoic Sharks. Anatom. Anzeig. 1896, 11, p. 678. 



