56 NEW YORK STATE MUSEUM 



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 Cladoselache are the oldest known structures 

 of the 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 con- 

 tinuous fold of skin, strengthened by parallel cartilaginous rods extending 

 outwards from the body wall, this fold becoming subdivided into the pairs 

 of pectoral and pelvic (ventral) fins as we know them in modern forms. 



Now it has been shown by Dr Bashford Dean that in Cladoselache 

 the paired fins were mere balancers with a more extended base line than is 

 usual. The series of parallel cartilaginous rods, which in a primitive con- 

 dition supported the lateral fin fold, exist practicall)' unmodified in the 

 pelvic fins, simply clustered and parll)- fused within the bod)- wall of the 

 pectoral fins. Uean, Cope, and others are of the opinion that there is a 

 tendency in the pectoral fin for the hinder end of the row of basals to 

 rotate outwards — a process which would reduce the point of attachment 

 of the fin to what was originally its front angle. The outwardly turned row 

 of basals would in this case correspond with the median axis of the well 

 known paddle in Ceratodus and Pleuracanthus, and one may without diffi- 

 culty conceixe of a fringe of cartilaginous rays developing cjuite secondarily 

 along the hinder border of this axis. Hence, as argued by Dean and Cope, 

 the modern tribasal or dibasal shark's fin can not have evolved from the 

 paddlelike "archipterygium," but these two kinds of fin must have arisen 

 independently from the " ptychopterygium," as the arrangement has been 

 appropriately termed by Cope.' 



' For interesting discussions of this subject, one may consult the following: Mollier, 

 S. Die paarigen Extremitiiten der Wirbeltiere. I. Das Ichthyopterygium. Anatom. Hefte, 

 3: 1893; Dean, B. Contributions to the Morphology of Cladoselache. Jour. Morphol. 1894. 

 9: 87-114; A new Cladodont from the Ohio Waverly. New York Acad. Sci. Trans. 1894. 

 13: 1T5-119; The Fin-fold Origin of the Paired Limbs, in tlie Light of the Ftyc;hopterygia 

 of Palaeozoic Sharks. Anatom. Anzeig. 1896. 1 1 : 673-79; Woodward, A. S. The Problem 

 of the Primeval Sharks. Nat. Sci. 1895. 6:38-43; Ibid. 1892. i : 28-35. 



