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 he 

 reduced to this type. Fusion in the radials has reduced these 

 jointed rods to a compressed mosaic of polygonal plates, — the 

 tin stem is consolidated into a basal cartilage band ol three 

 prominent elements, fore, middle, and aft. The dermal fin margin 

 has greatly encroached, and the component raj's 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 

 2>rotruded and jointed fin stem has acquired in all essential 

 characters the archipter^^gium of dipnoan and ancient crossoji- 

 terygian ; — the concentration and splitting of the radials is 

 carried to its utmost si3ecialization, the tendency to concrescence 

 distall}'- has caused them to spread around the fin tip and be 

 carried proximall}^ in their develojiment 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 Geratodus. 



While palaeontology has rendered material aid in the under- 

 standing the mode of origin of shark and di^Dnoan 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 " Cladoclus," 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 jDrobable that in this region the 

 concrescence, fusing, and splitting of the radial elements would 

 give rise to a:i archipterygium.-^ In this view, however, Smith 

 Woodward is directly opposed in a recent pajjer by Jaekel,"}" 

 who had also examined the type specimens in the mus- 



*L. c. 



t Sitz. Ber. der Gesell. nat. Fr., Berlin, 1892. No. G, p. 92. 



