| OCR Text |
Show 18870 PAIRED FINS OF CERATODUS. 13 less marked than with the pelvic member; but the fact which stands out most clearly is that the second mesomere invariably bears the greatest number of these rays. They are carried (figs. 5 and 6, mt.) upon a special lobe of the axial cartilage (" das divergirende Stiick " of Schneider) already alluded to. The free border of this lobe slopes, in every case examined by me, gradually towards the proximal mesomere (m.p.), and it is, moreover, in all, marked off from the body of the second mesomere by a deep furrow (indicated in the figs, by a dotted line). I now proceed to discuss its homology, and having arrived at the conclusions to be formulated in the sequel through a comparison with the pectoral fin of Cestracion, I pass at once to the consideration of that. Gegenbaur and Huxley are both agreed that the base of the Cestracion fin is supported by two cartilages (fig. 10) held by them to represent the mesopterygium (ms.) and the metapterygium (mt.) of other Selachians. Most recent writers have adopted their views (cf. Hubrecht and Sagemahl in Bronn's ' Klassen und Ordnungen des Thier-Reichs,' vol. vi. part 4, Pisces). Huxley, instituting a comparison (19, p. 50) between the corresponding fins of Cestracion and Notidanus, regards them as representative of the transition stages in the shortening of the CeratodusAike " archipterygium," by which he concludes the typical fish-fin has arisen. Gegenbaur (9, p. 148) likens the Cestracion fin to that of Acanthias, and says " das Propterygium fehlt ganzlich." Huxley, holding further that the propterygium (preaxial ray) of Cestracion is removed from the shoulder-girdle, as in Ceratodus, asserts that in Scyllium (pp. 50-52) " the further shortening of the axis gives rise to still greater changes. The axial cartilage (mesopterygium) is relatively small; but the enlarged postaxial cartilage (metapterygium) has extended upwards along the postaxial face of the first, until it has not only reached the articular surface of the pectoral arch, but furnishes a large part of the articular cavity. In like manner the proximal preaxial ray (propterygium) has ascended along the preaxial face of the axial cartilage, until it also is able to furnish a facet which completes the anterior part of the cup for the condyle of the pectoral arch." He holds therefore that the pectoral fins of Notidanus, Cestracion, and Scyllium represent, in the order enumerated, the successive steps in the modification alluded to above, and he, in accordance with the statements quoted concerning the propterygium and metapterygium, relegates the two former fins to his category of the " unibasal" type, as distinguished from that of the latter animal, which he holds to typify the " tribasal" one predominant among the Plagiostomes (cf. table which accompanies his essay). From an examination of the fins of two young Cestracions, I can state without further hesitation that the mesopterygium of the adult is (as Mivart has suspected, 21, p. 477) a compound of the pro- and mesopterygia. Fig. 9 represents one of the fins referred to. The animal died at the period at which the two (ms. and pt.) |
