Pectoral fins and girdle of an adult of Scyllium canicula (natural size, seen from behind and above).
co. Coracoid. sc. scapula. pp. propterygium. mep. mesopterygium. mp. metapterygium. fn. part of fin supported by horny fibre.
Fig. 2.
Right pelvic fin and part of pelvic girdle of an adult female of Scyllium canicula (natural size).
il. iliac process. pn. pubic process, cut across below. bp. basipterygium. af. anterior cartilaginous fin-ray articulated to pelvic girdle. fn. part of fin supported by horny fibres.
The true skeleton consists of three basal pieces articulating with the pectoral girdle; on the outer side of which there is a series of more or less segmented cartilaginous fin-rays. Of the basal cartilages one (pp) is anterior, a second (mep) is placed in the middle, and a third is posterior (mp). They have been named by Gegenbaur the propterygium, the mesopterygium, and the metapterygium; and these names are now generally adopted.
The metapterygium is by far the most important of the three, and in Scyllium canicula supports 12 or 13 rays[494]. It forms a large part of the posterior boundary of the fin, and bears rays only on its anterior border.
The mesopterygium supports 2 or 3 rays, in the basal parts of which the segmentation into distinct rays is imperfect; and the propterygium supports only a single ray.
The pelvic fins are horizontally placed, like the pectoral fins, but differ from the latter in nearly meeting each other along the median ventral line of the body. They also differ from the pectoral fins in having a relatively much broader base of attachment to the sides of the body. Their cartilaginous skeleton (woodcut, fig. 2) consists of a basal bar, placed parallel to the base of the fin, and articulated in front with the pelvic girdle.