In such an archaic Selachian as _Pleuracanthus_ the fin is clearly of
the biserial archipterygial type, but the lateral rays are reduced
(pectoral) or absent (pelvic) (fig. 17, a) on one side of the axis. In a
typical adult Selachian the pectoral fin skeleton has little apparent
resemblance to the biserial archipterygium--the numerous outwardly
directed rays springing from a series of large basal cartilages (_pro_-,
_meso_- and _metapterygium_). The condition in the young (e.g. fig. 17,
b, _Acanthias_) hints strongly, however, at the possibility of the fin
skeleton being really a modified biserial archipterygium, and that the
basal cartilages represent the greatly enlarged axis which has become
fixed back along the side of the body. In Crossopterygians
(_Polypterus_) the highly peculiar fin skeleton (fig. 18) while still in
the embryonic cartilaginous stage is clearly referable to a similar
condition. In the Actinopterygians--with the increased development of
dermal fin rays--there comes about reduction of the primitive limb
skeleton. The axis becomes particularly reduced, and the fin comes to be
attached directly to the pectoral girdle by a number of basal pieces
(Teleosts) probably representing vestigial rays (cf. fig. 19).
Views on the general morphology of the fin skeleton are strongly
affected by the view held as to the mode of evolution of the fins. By
upholders of the lateral fold hypothesis the type of fin skeleton
described for _Cladoselache_[30] is regarded as particularly
primitive. It is, however, by no means clear that the obscure basal
structures figured (Fig. 20) in this fin do not really represent the
pressed back axis as in _Pleuracanthus_.
The pelvic fin skeleton, while built obviously on the same plan as the
pectoral, is liable to much modification and frequently degeneration.
[Illustration: From Bashford Dean, Mem. _N.Y. Acad. of Science_.