The pectoral fins of Elasmobranchs vary very much in their mode of attachment. In some of the sharks, including the Notidanidae and Scyllium, all three basalia articulate with the pectoral girdle, while in others such as Cestracion the meta-pterygium is excluded. In Rays the propterygium and the meta-pterygium are long and narrow and diverge much from one another; other basalia work their way in between the meso-pterygium and meta-pterygium, and come to articulate with the pectoral girdle. Sometimes they fuse and form a second meso-pterygium. The radiale are greatly elongated and are segmented.
In Chimaera all three basalia are present, but the meso-pterygium is shifted and does not articulate with the pectoral girdle[49].
In Acipenser and Polyodon the pectoral fin is built on the same type as in Elasmobranchs, but becomes modified from the fact that the propterygium is replaced by dermal bone which forms a large marginal ray. Extra meso-pterygia are formed in the same way as in Rays.
In Polypterus the pro-and meta-pterygia have ossified while the meso-pterygium remains chiefly cartilaginous; the fin-rays are also chiefly ossified.
In Amia, Lepidosteus, and certain Teleosteans like Salmo, not only the propterygium but the meso-pterygium is almost suppressed by the marginal ray.
In the great majority of Teleosteans a still further stage is reached, the endoskeletal elements, the basalia and radiale are almost entirely suppressed and the fin comes to consist entirely of ossified fin-rays of dermal origin.
In some Teleosteans—Exocaetus, a herring, and Dactylopterus, a gurnard—the pectoral fins are so enormously developed that by means of them the fish is able to fly through the air for considerable distances. The skeleton of these great fins is almost entirely composed of dermal bone.
Pelvic fin.
The pelvic fin is almost always further removed from the archipterygial condition, and is in general more modified than is the pectoral. Thus in the Ichthyotomi, while the pectoral fins are archipterygia similar to those of Ceratodus, the pelvic fins consist of an axis bearing rays on the postaxial side only, and prolonged distally into a clasper. In Dipnoi however the pelvic fins are very similar to the pectoral. In Elasmobranchs the meso-pterygium is missing, the propterygium is small or absent, and the fin is mainly composed of the meta-pterygium (generally called basi-pterygium) and its radiale. The males in Elasmobranchii and Holocephali have the distal end of the meta-pterygium prolonged into a clasper.
In Ganoids and in Teleosteans the loss of the pelvic girdle causes the pelvic fin to be still further removed from the primitive state. There is always a large basi-pterygium which lies imbedded in the muscles and meets its fellow at its proximal end. In Cartilaginous Ganoids it has a secondary segmentation. Its relation to its fellow is subject to much variation in Teleosteans, sometimes as in the Perch the two are in contact throughout, sometimes as in the Salmon they meet distally as well as proximally, but are elsewhere separated by a space, sometimes as in the Pike and Bony Ganoids they diverge widely. The radiale are articulated to the basi-pterygium. In Cartilaginous Ganoids and Polypterus they are well developed, in other Ganoids and in Teleosteans they are in the main replaced by dermal fin-rays.