| OCR Text |
Show 46 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, III. The Pectoral Limb. In the memoir cited, Dr. Gunther describes the limbs of Ceratodus and their skeleton as follows :- " The limbs consist of two pairs of paddles, similar in appearance to the termination of the tail; viz. a longitudinal axis, formed by the endoskeleton and muscles and covered with scales, is surrounded by a broad rayed fringe. These paddles are structurally identical with the fins of Lepidosiren; only the axis and also the fringe are much dilated. The pectoral and ventral paddles taper to a fine point, the former being longer than the head, the latter rather shorter. The ventral paddles are inserted at a short distance in front of the vent" (p. 515). " The paddle is joined to the scapular arch by an elongate, flattish, slightly curved cartilage ; its proximal end has a glenoid cavity, fitting into the humeral condyle ; the joint is simple, free, allowing of a considerable amount of motion, its parts being held together by a ligament fastened round its circumference. This is the only true joint in the limb, all the other parts being fixed to one another by connective tissue. I consider this cartilage to be the forearm ; a horizontal section along its longitudinal axis does not show any primary division. The next following cartilage forms the base of the paddle ; although externally it appears as a single flat, broad, short piece, unevennesses of its surface indicate that several primary pieces are coalesced in it. " I am confirmed in this view by a horizontal section, in which the lines of the former divisions are preserved in the shape of tracts of a white connective tissue. Three such divisions may be distinguished, corresponding to the three carpals of most Plagiostomes *. If this determination is correct, then the antibrachial cartilage just described is not represented in that order. " The remaining framework of the paddle shows an arrangement unique among the Vertebrata. From the middle of the basal cartilage a series of about twenty-six subquadrangular pieces takes its origin, forming a longitudinal axis along the middle of the paddle to its extremity. The pieces become gradually smaller, and are scarcely distinguishable towards the end of the paddle. On the two posterior corners of each piece a branch is inserted, running obliquely backwards towards the margin of the fin; the branches of the first eight or twelve pieces are three-jointed, the remainder two-jointed, the last having no branch at all. Slight irregularities, such as the origin of two branches from one side of a central piece, occur, as also several four-jointed branches being inserted immediately on the basal cartilage " (pp. 532-3). In general, this description suits the pectoral fins of the specimen I have described very well. Mine, however, has only twenty median cartilages. All but the very last bear lateral rays ; but towards the distal end of the fin these become minute, and consist of a single piece. Moreover the distal joints are much more slender, especially Pro-, meso-, and metapterygium of Gegenbaur. |
