122 TRANSACTIONS OF THE [Frs. 23 
cartilage rod, “basal” firmly inserted in the body tissues; there 
may here be disregarded the questions of additional segmen- 
tation proximally 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 
origin, This stage inthe evolution of the finis clearly suggested 
in the pelvics of Xenacanthids and in the almost as primitive 
pelvies 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, whose 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 specialized fin structures, 
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, which were formerly rod-like and. parallel, 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 
fittinely then could a form 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 body 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 split 
in the region of the distal fin stem. The derm margin of the 
