. The elasmobranch fishes . A B rig. 84. Pectoral girdles. A. Heterodontus francisci. B. Torpedo. (Modified from Gegen- baur.) CO., coracoid; , aperture for nerve and blood vessels to pectoral fin; sc, scapula. is like that of the rays in extent; it has, however, a much heavier mesopter- ygium than have the rays. Usually the propterygium of the rays is divided into a number of segments which extend forward to join the antorbital process {Bhinohatis, fig. 83, ; Raja, Myliohatis), or even to the tip of the ethmoid region where the two from the opposite sides unite (Urolophus). T
. The elasmobranch fishes . A B rig. 84. Pectoral girdles. A. Heterodontus francisci. B. Torpedo. (Modified from Gegen- baur.) CO., coracoid; , aperture for nerve and blood vessels to pectoral fin; sc, scapula. is like that of the rays in extent; it has, however, a much heavier mesopter- ygium than have the rays. Usually the propterygium of the rays is divided into a number of segments which extend forward to join the antorbital process {Bhinohatis, fig. 83, ; Raja, Myliohatis), or even to the tip of the ethmoid region where the two from the opposite sides unite (Urolophus). The mesop- terygia of the rays are very variable. In some forms a mesopterygium is absent whereupon the radials extend to the girdle. In other types there is a consider- able mesopterygial plate, as in Rhinohatis {, fig. 83). In others still, a second plate back of the mesopterj'^gium may be formed, as in Pteroplatia. The metapterygium of the rays, like the propterygium, is greatly developed, pass- ing backward to the region of the pelvic fin. The propterygial radials of the sharks are usually few in number. They may form a single line of segments which may be of more or less regular plates, as in Heptanchus or Heterodontus francisci (fig. 82). In others the rows may be more numerous. In the rays there are many rows of such propterygial radials, some of which are made up of great numbers of segments (Rhinobatis, fig. 83). The radials attached to the mesopterygium in the sharks are more numerous
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