1893.] NEW YORK ACADEMY OF SCIENCES. 121 



Note on the Mode of Origin of the Paired Fins. 



by bashford deax. 

 (abstract.) 



Since the time of the observations of Balfour on the origin 

 of the paired fins from continuous lateral derm folds, an exten- 

 sive literature upon the subject has been steadily accumulating-. 

 At the present day it is very generally admitted that this mode 

 of origin is well substantiated by embryological studies upon 

 elasmobranchs, and in the " Gliedmassenskelet" of Wiedersheim 

 the most recent view is given tracing from this primitive form 

 the evolution of the various limb tyjies, Archiplerycjiam, 

 Actinopterygium and Cheiropterygium. 



It is especially interesting that within the past five years the 

 actual primitive conditions in fin structure are coming to be 

 provided by paleeontology, and these now appear to directly 

 confirm the conclusions obtained on the side of embryology. 

 The earlier fossil fin structures can no longer be looked upon as 

 in a measure supporting the archipterygial doctrine of 

 Gegenbaur, 



In a recent article Smith "Woodward* has reviewed the 

 evolution of fins in the light of his studies upon fossil forms, 

 and shows how perfect is the evidence which reduces the 

 structural characters of the paired fins to the type of the 

 unpaired fins, on the ground of concentration and fusion of the 

 supporting elements. These processes are first manifested in 

 the basal parts and subsequently are produced into the rachis 

 of the fin. The important contribution to the problem has 

 been the structural characters in the pectoral and ventral of 

 the Carboniferous Xenacanthids as studied by Fritsch, 

 Doderlein, and Wiedersheim. In Xenacathids the pectoral is in 

 all essentials dipnoan in character, while the ventral, always 

 more primitive in structure, retains a typical monoserial 

 archipterygium. 



The most ancient form of fin suggested by these studies 

 appears to be represented by a lateral derm fold not unlike the 

 embryonic structure of sharks, — this was primitively strength- 

 ened by parallel hair-like rays, trichinosts, passing from body Avail 

 to fin margin, developed in close connection with ectoderm from 

 mesoderm (Ryder). In this'derm fold cartilaginous rays make 

 their appearance, "radials," passing rod-like from fin margin to 

 body wall, each of which is attached primitively to a similar 



— 



♦Natural Science, March, 1892. 



