1887.] 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 Cestracio7i, I pass at once to the considera- 

 tion 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 (?)is.) 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 Ceratodus-like "archipterygium," 

 by which he concludes the typical fish-fin has arisen. Gregenbaur 

 (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 (meso- 

 pterygium) 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 pec- 

 toral 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 con- 

 cerning 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 accom- 

 panies 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.) 



