1887.] PAIRED FINS OF CERATODUS. 17 



receive refutation, proportionate to the support furnished for those 

 more especially of Balfour, Haswell, and Dohrn already cited. 



As stated previously, the cartilage r of fig. 1 is ray-like, but 

 stouter and more powerful than that of any ordinary paramere. 



In seeking light on this question, one naturally turns to Poly- 

 pterus, the aifinities between which and the Dipnoi, originally 

 pointed out by Huxley (18), have nowhere been denied. The 

 Polypterus pectoral fin is, as is well known, supported upon three 

 basal elements. The mesopterygium (fig. 11, ms.) is held by 

 all to represent that of the Plagiostomes, and no oue has yet 

 challenged Gegenbaur's determination (9, p. 148) of the homology 

 between the elongated postaxial bar (jnt.) of this fish and the 

 metapterygium of the Plagiostomes and Chimceroids. Huxley says 

 of this fin (19, p. 53) that "the Scyllium type is essentially 

 preserved." Comparison of the Polypterus pectoral fin (fig. 11) 

 with the pelvic fin of Ceratodus represented in fig. 1 would appear 

 at first sight to suggest a homology between the basal postaxial bar 

 (r) of the latter and the metapterygial bar (mt.) of the former. 

 If this be justified, it would further appear, accepting the homology 

 of the metapterygium of Polypterus with that of the Elasmo- 

 branchii, that the two fins might have been derived along a line of 

 modification characterized by the assumption on the part of the 

 metapterygium of a ray-like character, and by the subsequent 

 elongation of the mesopterygial plate {ms.). The probable truth 

 of the latter assertion seems to me very great indeed. The meso- 

 pterygium is, in Pohjpterus (fig. 11, ms.), already elongated beyond 

 the limits met with elsewhere, displacing in the process the mar- 

 ginal rays. Continue that elongation, and there could only result a 

 Ceratodus-\\ke product. As concerns the former supposition, how- 

 ever, comparison of the fin-skeletons represented in figs. 1, 3, & 4 is 

 sufl3cient in itself to show that the proximal postaxial ray of fig. 1 most 

 probably represents the distal one of those related to the proximal 

 mesomere of fig. 4. Comparison of the latter (fig. 4) with the 

 proximal end of the pectoral fin cf the same side of the same animal 

 (fig. 8) shows unmistakably that in the plate-like structure result- 

 ing from the fusion of the basal ends of the two proximal para- 

 meres we have to deal with the last trace of the metapterygium, 

 defining that, as must now be done, as a product of the confluence 

 of the inner ends of the proximal postaxial rays, the distal ray being, 

 from the nature of its relations therewith, one of the same series. 



Consideration of the above facts renders the homology of the sup- 

 posed metapterygium of Polypterus somewhat doubtful. Gegen- 

 baur, when pointing to the same, realized the similarity between both 

 pro- and metapterygia so-called by him (fig. \\,pt. andrn^.) and the 

 marginal rays'. He at first suggested (10, p. 139) the possibiUty 

 that the exclusion of the mesopterygium from connexion with the 



^ The difficulty of interpretation of the supposed propterygium is greatly 

 increased by the presence of the cartilage marked * in fig. 11, — by no means the 

 least puzzling element in this fin. As wiU be seen, it is grafted upon the ante- 

 rior border of the propterygial rod ; from it, however, it is perfectly distinct in 



Proc. Zool. Soc— 1887, No. II. 2 



