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Show 1878.] PROF. MIVART ON THE FINS OF ELASMOBRANCHS. 119 lateral fold so as to form two prominences on each side, i. e. the primitive paired limbs. (4) Each anterior 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 (as in Pristis &c.) adhesion to the skeletal axis. (7) The rays of the hinder paired limbs did so more constantly, and ultimately prolonged themselves inwards by mesiad growths from their coalesced bases, till the piscine pelvic structure arose as we see it in Squatinal (8) The pectoral rays 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 axis of the digit-bearing limb. (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 as 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 in those cases where its function was quite similar to that of the pectoral limb its structure became also quite similar (e. g. Icthyosaurus, Plesiosaurus, Chelydra, & c ) ; but for the quadrupedal mode of progression it became segmented and inflected in a way generally parallel with, but (from its mode of use) in most cases in part inversely to, the inflections of the pectoral limbs. The amount of apparently spontaneous change needed to effect these transformations may appear excessive ; but I believe that the excessive plasticity of the animal organism is generally too little appreciated- a plasticity which results in and is evidenced by the many instances we now know of the independent origin of similar structures. The plasticity of animals might be expected to be great; for plasticity is bodily reaction in response to external stimuli. The response which is most rapid and complete is sensation ; and an animal is a creature 1 This view has been put forward by Mr. J. K. Thacher of New Haven, Connecticut. See Connecticut Trans, vol. iii. |