. The elasmobranch fishes. Chondrichthyes. THE ELASMOBRANCII FISHES 77 radials {ra., fig. 82). In some forms there may also be found (3) a series of distal plate-like radials {Heterodontus) between upper and lower dermal rays. A knowledge of the development of such a skeleton is helpful to an under- standing of the adult frame- work. In Scyllium canicula (fig. 81) Balfour (1881) has shown that the pectoral skele- ton arises as a horizontal bar or jilate of cartilage from which radials (ra., fig. 81) extend. These radials by fusion at their outer tips form a rim from which plate-like distal rad
. The elasmobranch fishes. Chondrichthyes. THE ELASMOBRANCII FISHES 77 radials {ra., fig. 82). In some forms there may also be found (3) a series of distal plate-like radials {Heterodontus) between upper and lower dermal rays. A knowledge of the development of such a skeleton is helpful to an under- standing of the adult frame- work. In Scyllium canicula (fig. 81) Balfour (1881) has shown that the pectoral skele- ton arises as a horizontal bar or jilate of cartilage from which radials (ra., fig. 81) extend. These radials by fusion at their outer tips form a rim from which plate-like distal radials ]iass well out into the fin. As growth progresses the original bar of cartilage becomes the main axis of the fin skeleton, the metapterygium {, fig. 81a) to which numerous ra- dials are attached. Anterior to the metapterygium, the so- called median piece or mesop- terygium () arises sec- ondarily; fewer radials pro- ject from it. There is next seg- mented off from the anterior part of the mesopterj-gium a piece, the propterygium (, fig. 81b), which bears a single plate-like radial. As for the pectoral girdle, it is formed secondarily from the anterior tip of the horizontal bar. In the adult shark the sim- plicity of plan characteristic of this embryonic fin is rarely retained (Chlamj/doselachus), yet the fundamental plan here laid down is the same, even in the most specialized of pec- torals. Propterygium, meso-, and metapterygium are usu- ally present. The propteryg- Fig. 83. The adult pectoral fin, Ehinohatis produc- • ' \. n ^ -ii li tus. (Mildred Bennett, del.) mm may be fused with the J^_^ mesopterygium; , metapterygium; meS0])teryginm. as in the adult , neopterygium; , Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Daniel, J. Frank (John Franklin),
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