4^ 



NATURAL SCIENCE. 



Jan., 



regard it at present merely as a convenient guide for further investi- 

 gation. The present writer is by no means convinced that it better 

 explains the known facts than the theory to which he expressed 

 adherence in 1892 (9). It is definitely known that among fringe- 

 finned fishes the originally long lobate fin gradually become sshortened ; 

 and the abbreviate lobate pectoral fin of the modern representative 

 of the order (Polyptevus) differs in no essential respects from the 

 corresponding fin of a typical modern shark. The facts of Palaeon- 

 tology, as at present] understood, still seem rather to favour the 

 idea that the same kind of evolution has taken place among 

 Elasmobranchs. 



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

 the paired fins of Cladoselache (Fig. 2) can be compared with those of 



Fig. 3. — Acanthodes bronni ; outline restoration by Dr. A. Fritsch (4). Lower 

 Permian, Germany. 



an Acanthodian (Fig. 3). We venture to maintain that these fins 

 are fundamentally different in every respect. In Cladoselache the car- 

 tilages of the internal skeleton are well developed and support 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 (3) speaks of the " radials " of 

 Cladoselache as if, by fusion, they might readily become a fin-spine like 

 that of the Acanthodian Parextts ; but the former are cartilage and 

 endoskeletal, the latter is merely the ordinary dentine and therefore 

 presumably exoskeletal. The problem of the primaeval 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 Devonian 

 Holoptychius. Everything still tends to show that the very highest 

 Elasmobranchs lived simultaneously with almost the lowest in late 

 Palaeozoic times ; while sharks and skates nowadays are a com- 

 paratively degenerate race. 



REFERENCES. 



1. Cope, E. D. — "New and little-known Palaeozoic and Mesozoic Fishes." Journ. 



Acad. Nat. Sci. Philad., vol. ix., pp. 427-431 ; 1894. 



2. Dean, B. — " Contributions to the Morphology of Cladoselache." Journ. Morphol., 



vol. ix., pp. 87-114 ; 1894. 



3. . — "A new Cladodont from the Ohio Waverly." Trans. New York Acad. 



Sci., vol. xiii., pp. 115-119; 1894. 



