RUN 
No. 430.] PAIRED LIMBS OF VERTEBRATES. 771 
whether or not we accept the origin of their supporting elements 
as equivalent to the coalesced radial supports of a more typi- 
cal shark-like fin. On the other hand, the paired fins of the 
cladoselachids are distinctly shark-like, although in pattern 
unlike those of later selachians. They are remarkable in that 
they are built on the plan of balancing organs; ze, the fin 
was attached keel-fashion, along the side of the body, and prob- 
ably functioned by means of ventrad and dorsad movements 
(undulatory at fin margin ?). And the blade of the fin could 
not be rotated (twisted) sideways. This is attested structur- 
ally by (1) its graded series of radials, — larger, most numer- 
ous, and most crowded in the anterior region ; (2) by the absence 
of a metapterygial lobe, and in the continuation of the fin web 
hindward along the margin of the body ; physiologically (3) by 
its close analogy with the dorsal and anal of such a form as 
Mola, in which we know that this undulatory movement has 
been specialized. That the paired fins of Cladoselache, more- 
over, cannot be interpreted as **due to bottom living " on the 
plan, e.g., of Rhina, becomes clear when we consider that the 
radial cartilages are unjointed throughout their length, that 
the ventral and pectoral fins are not drawn together in a ray- 
like way, and that the fins, in spite of their remarkable charac- 
ter, are as small in proportion to the size of the body as those 
of modern selachians (Pristiurus, Scyllium), which are known 
to be free swimming. (Cf. Natural Science, Vol. VIII (1896), 
pp. 250-252, and Anat. Anzeiger, Bd. XI, pp. 673-679. Vide 
also infra.) 
III. Tue Evipence or PALEONTOLOGY AS TO THE DEVEL- 
OPMENT OF THE UNPAIRED FINS. 
In recent work tending to support the hypothesis of Gegen- 
baur no reference is made to the mode of origin of the 
unpaired fins. This, we must grant, would have an important 
bearing upon the problem of the extremities if evidence existed 
to show that in a late stage in the evolution of the unpaired 
fins an archipterygium arose, me that of the paired fins. As 
a matter of fact, paleontol trates that such a condition 
oy 

