122 TRANSACTIONS OF THE [FeB. 23 



cartilage rod, "basal" firmly inserted in the body tissues; there 

 may here be disregarded the questions of additional segmen- 

 tation prosimall}^ and of the continuity of radials throughout the 

 length of the fold. When concrescence takes place the basals 

 become fused into a lateral horizontal bar of cartilage, 

 more or less segmented in character ; from this trunk of basals 

 embedded in the body wall, the radials are seen to take their 

 oi'igin. This stage in the evolution of the fin is clearly suggested 

 in the pelvics of Xenacanthids and in the almost as primitive 

 pelvics of the cartilaginous ganoids, notably in Polyodon. The next 

 stage of the evolution is represented by the gradual out turning 

 of the trunk of the basals, v^hose posterior terminal comes to 

 protrude from the body wall, and whose anterior end tends to 

 become proximal. This out-turning of the basal fin stem brings 

 with it a most important change in the functional capacity and 

 indicates the point of divergence for s^iecialized fin strvictures. 

 Up to this point the entire fin was but a compressed remnant of the 

 lateral fold, whose line of motionwas little more than dorso-ventral. 

 The protruding distal end of the trunk of basals now becomes 

 the fin stem and as its motion becomes developed in many planes 

 two prominent characters become evolved, — a tendency to 

 concentration of elements about the distal fin stem, and second a 

 growth of the dermal margin of the fin. The first causes the 

 radials, Avhich were formerly rod-like and j^arallel, to become 

 so concentrated that extended fusions take place, and appears 

 to be the cause of the jointed character that the rays now 

 present, perhaps also of the branching and splitting structure 

 of terminal elements, including doubtless those of the fin stem 

 itself. The second character acquired by a fin in this evolution 

 is the development concomitantly of a wide fin margin strength- 

 ened by dermal rays. The extended fin surface becomes 

 doubtless of great advantage as the fin requires its additional 

 movements, — and the formation, neomorphic (?), of light strong 

 horn-like derm rays answers no doubt this requisite more 

 fittingly then could a fo]-m of specialization of the cartilaginous 

 radials. It would in fact appear in further evolution that the 

 radials become reduced, encroached upon and outmatched in 

 function. 



A fin type represented by a jointed fin stem protruding from 

 the bod}^ wall, articulating with the basals therein remaining\ 

 furnished on one side with a row of rod-like radials, is the 

 actual condition in the pelvics of Xenacanthids, or cartilaginous 

 ganoids. The radials are, however, becoming specialized; they 

 become jointed and are tending to concentrate, fuse or sj^lit 

 in the region of the distal fin stem. The derm margin of the 



