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Show 658 MR. F. M. BALFOUR ON THE SKELETON [June 7, angles with the long axis of the body, were developed in varying extents in all these four longitudinal folds. "3. The longitudinal folds became interrupted variously, but so as to form two prominences on each side, i. e. the primitive paired limbs. "4. Each anterior paired limb increased in size more rapidly than the posterior limb. "5. The bases of the cartilaginous supports coalesced as was needed, according to the respective practical needs of the different separate portions of the longitudinal folds, i. e. the respective needs of the several fins. " 6. Occasionally the dorsal radials coalesced (as in Notidanus &c.) and sought centripetally (Pristis &c.) adherence to the skeletal axis. " 7. The radials of the hinder paired limb did so more constantly, and ultimately prolonged themselves inwards by mesiad growth from their coalesced base, till the piscine pelvic structure arose, as, e. g., in Squatina. " 8. The pectoral radials with increasing development also coalesced proximally, and thence prolonging themselves inwards to seek a point d'appui, shot dorsad and ventrad to obtain a firm support, and at the same time to avoid the visceral cavity. Thus they came to abut dorsally against the axial skeleton, and to meet ventrally together in the middle line below. " 9. The lateral fins, as they were applied to support the body on the ground, became elongated, segmented, and narrowed, so that probably the line of the propterygium, or possibly that of the mesopterygium, became the cheiropterygial axis. " 10. The distal end of the incipient cheiropterygium either preserved and enlarged preexisting cartilages or developed fresh ones to serve fresh needs, and so grew into the developed cheiropterygium; but there is not yet enough evidence to determine what was the precise course of this transformation. "11. The pelvic limb acquired a solid connexion with the axial skeleton (a pelvic girdle) through its need of a point d'appui as a locomotive organ on land. "12. The pelvic limb became also elongated; and when its function was quite similar to that of the pectoral limb, its structure became also quite similar (e. g. Ichthyosaurus, Plesiosaurus, Chelydra, &c.); but for the ordinary quadrupedal mode of progression it became segmented and inflected in a way generally parallel with, but (from its mode of use) in part inversely to, the inflections of the pectoral limb." Gunther1 has propounded a theory on the primitive character of the fins, which, on the whole, fits in with the view that the paired fins are structures of the same nature as the unpaired fins. The interest of Giinther's views on the nature of the skeleton of the fins more especially depends upon the fact that he attempts to evolve the fin 1 " Description of Ceratodus," Phil. Trans. 1871. |