i8 9 6. SHARKS AS ANCESTRAL FISHES. 253 



Even its specialisations are evidently of a lowly order : the differentia- 

 tion of the fins, their radial supports, with their range of size and 

 shapes clustering in the anterior fin-margin, are of this class ; also 

 the shapes of the cladodont teeth, and the large shagreen denticles sur- 

 rounding the orbits or strengthening the cut-water margins of the fins; 

 as well as the curious differentiation of the caudal fin, in its extreme 

 degree of heterocercy and in its dermal lateral keels. The sum of the 

 generalised features of this most ancient shark cannot, moreover, fail 

 to place it, judging from the standpoint of the morphologist, near the 

 ancestral stem of the Elasmobranchii. Its primitive fin-characters 

 and lack of clasping organs, moreover, break one of the strongest 

 barriers that have separated the primitive phylum of the Elasmo- 

 branchii from that of the Teleostomi and lung-fish. Furthermore, it 

 is with the characters of Cladoselache that the puzzling structures of 

 the Acanthodians are now most reasonably to be compared. The 

 fin-structures of the more generalised form of Cladoselache are found 

 to be clearly approaching those of the Acanthodians. The fin-spines 

 of the latter, although clearly encased outwardly with purely dermal 

 structures, 1 are, nevertheless, to be regarded as of compound origin, 

 representing morphologically the concrescence of the radial supports 

 in the anterior margin of the fin ; and it is significant that the fin 

 of a Cladoselachid, which has been discovered by, and is now in 

 the collection of, Dr. Clark, 2 is decidedly of a spine-like character, 

 nearly thrice as long as wide, its supporting elements forming a 

 compact and Acanthodian-Iike fin-support. 



Up to the present time the discoveries among the primaeval 

 sharks can only be regarded as the beginnings of our knowledge of 

 these remarkable forms. As beginnings, however, they must certainly 

 be acknowledged as already of extreme interest, demonstrating the 

 existence of elasmobranchs of primitive and generalised characters, 

 and enabling us to form an idea of what may have been the direct 

 ancestors (" Pvoselachia") of the recent sharks and rays, if not, indeed, 

 of the remaining groups of fishes. 



Bashford Dean. 

 Columbia College, New York. 



1 Cf. Smith Woodward, Natural Science, vi., pp. 38-44. 



2 By whose courtesy the writer is enabled to describe and figure it in a forth- 

 coming number of the Anatomischev Anzeiger. 



