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_. FIG. 20.--Skeleton of Pectoral Fin of _Cladoselache_.] [Illustration: FIG. 21.--Placoid elements of a male Thorn-back, _Raia clavata_.]