1893. | NEW YORK ACADEMY OF SCIENCES. 121 
Note on the Mode of Origin of the Paired Fins. 
BY BASHFORD DEAN. 
(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 lmb types, Archipterygium, 
Actinopterygium and Cheiropterygium. 
Itis especially interesting that within the past five years the 
actual primitive conditions in fin structure are coming to be 
provided by paleontology, 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 bas 
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 
archiptery gium. 
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 wall 
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. 
