. Elements of Comparative Anatomy. 478 - COIMPAEATIVE ANATOIMY. lateral rays are considerably developed. The remains of tlie medial row are ordinarily quite small (Fig. 263, B'), but they are always sufficiently distinct to justify us in sup- posing that in higher forms the two sets of rays might be better developed. Rays are still attached to the stem, and are connected with the shoulder-girdle by means of larger plates (p ms). The joints of the rays are sometimes broken up into polygonal plates^ which may, further, fuse with one another; concres- cence of this kind may also affect the pieces
. Elements of Comparative Anatomy. 478 - COIMPAEATIVE ANATOIMY. lateral rays are considerably developed. The remains of tlie medial row are ordinarily quite small (Fig. 263, B'), but they are always sufficiently distinct to justify us in sup- posing that in higher forms the two sets of rays might be better developed. Rays are still attached to the stem, and are connected with the shoulder-girdle by means of larger plates (p ms). The joints of the rays are sometimes broken up into polygonal plates^ which may, further, fuse with one another; concres- cence of this kind may also affect the pieces which form the base of the fin {jy ms). By regarding the free rays, which are attached to these basal pieces, as belonging to these basal portions, we are able to divide the entire skeleton of the fin into three segments—pro-, meso-, and metapterygium. The metapterygium (mt) represents the stem of the archipterygium and the rays on it. The propterygium (p) and the mesopterygium (ms) are evidently derived from rays which still remain attached to the shoulder-girdle. The peculiar form of the fin in the Ray is due to the great development of the propterygium; the arrangement in Squatina leads towards this. One ray is here converted into a support for rays, and forms, by gradually reaching forwards, a stem for the propterygium, just as the metapterygium in the stem of the archipterygium possesses one. The Chimeeree agree in all essential points with the Fig. 263. Skeleton of the thoracic fin of Acanthias vulgaris, p Basale of the protopterygimn. mt Of the metapterygium. B Median edge of the fin. The lino drawn through mt indicates the series which formed the stem of the archipterygium. The dotted lines correspond to the rays, which are mostly arranged at the sides (R R), and are rudimentary only on the medial Bide (R'). § 362. The skeleton of the thoracic fin in the Ganoidei may be derived from a condition which is similar to that which obtains in the Shark; it is the
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