1893. ] NEW YORK ACADEMY OF SCIENCES. 123 
fin has, in addition, encroached upon the older fin elements, 
and constitutes more than half of the fin surface. 
The fin structure of the modern shark may readily be 
reduced to this type. Fusion in the radials has reduced these 
jointed rods to a compressed mosaic of polygonal plates, —the 
fin stem is consolidated into a basal cartilage band of three 
prominent elements, fore, middle, and aft. The dermal fin margin 
has greatly encroached, and the component rays have often 
grown and strengthened in the exposed (pre-axial) margin of 
the fin. In all of these specializations the ancient fin stem is 
coming to be directed caudad, and lies near the side in the 
direction of the axis of the body. 
The evolution of the second type of fin is readily understood 
in the structure of the pectoral of Xenacanthus,—here the 
protruded and jointed fin stem has acquired in all essential 
characters the archipterygium of dipnoan and ancient crossop- 
terygian ;—the concentration and splitting of the radials is 
carried to its utmost'specialization, the tendency to concrescence 
distally has caused them to spread around the fin tip and be 
carried proximally in their development on the opposite side of 
the fin stem; this itself in the process of concrescence has 
probably received distal increments. In this specialized type 
environment has had, doubtless, a large share in preserving 
the rigid axis in the middle of the fin, for uses which have been 
often alluded to in the case of Ceratodus. 
While paleontology has rendered material aid in the under- 
standing the mode of origin of shark and dipnoan fin structures, 
until recently it has afforded little clue to the still more primi- 
tive types. Smith Woodward has referred to a shark from the 
Ohio Waverly (Lower Carboniferous), described by Newberry 
and assigned provisionally to the genus ‘‘ Cladodus,’’ as exhibit- 
ing the most primitive fin structures extant, The paired fins 
he regards as functional remnants of the lateral folds, supported 
by nothing more than unjointed rods of cartilage passing from 
body wall to fin margin and apparently lacking in basal supports. 
He notes the significance of the concentration of the rays at the 
fore-margin of the pectoral fin, and in the absence of basal 
supports regards it as probable that in this region the 
concrescence, fusing, and splitting of the radial elements would 
give rise to an archipterygium.* In this view, however, Smith 
Woodward is directly opposed in a recent paper by Jaekel,f 
who had also examined the type specimens in the mus- 
*T.¢. 
1 Sitz. Ber. der Gesell. nat. Fr., Berlin, 1892, No. 6, p. 92. 
