1887.] PAIRED FINS OF CERATODUS. 23 



condition. Its postaxial border is supported by a cartilage, admitted 

 by all to represent the metapterygium (fig. 12, int.). This appears 

 to be produced out into a preaxial lobe, which is regarded by Davidoff 

 (4. p. 470, pi. 28. fig. 3, and pi. 29. fig. 18), who last described 

 it, as consisting of a single piece answering to the propterygium. 



It also recalls most closely that lobe from which Balfour held (1, 

 p. 667) that both pro- and mesopterygia are derived. In a young 

 Chimseroid pelvic fin examined by me (fig. 12), the lobe in question is 

 seen to be formed by the fusion of three preaxial rays, and careful 

 examination has shown that the last traces of an original separation 

 between it and the metapterygium (indicated in the drawing by a 

 dotted line) exist. Did that persist, the fin would correspond in all 

 essential respects with the^'pectoral member, as I have defined it ; and 

 I hold that this preaxial lobe is neither more nor less than the pro- 

 pterygium '. Mivart comments (21, p. 465) upon the "close 

 resemblance " between the pectoral and pelvic fins of the Chimseroids. 

 Comparing the pelvic fin of these animals {Callorhynchus) with the 

 pectoral ones o\ Acanthias aaA Scymnus, he concludes (p. 456) that 

 the basal cartilage represents, in the former, all three pterygia fused 

 into one. The considerations put forward above, taken together 

 with the fact that the mesopterygium never appears in the Plagio- 

 stome's pelvic fin, beyond the insignificant degree observed by 

 Haswell, appear to me to negative this view. 



The facts now under notice suggest, but do not prove, that the 

 mesopterygium is never represented at all in the Chimseroids ; and 

 that with respect to that feature those fishes stand on a lower platform 

 than do the living Plagiostomes. Moreover, if the preaxial cartilages 

 of their pectoral member represent the propterygium, as I believe, 

 an absolute structural identity is proven between the pectoral and 

 pelvic fins of the group. Both would appear to have been derived 

 from the fins of an ancestor in which the mesopterygium was not 

 differentiated ; and if so, that element must have been of compara- 

 tively late origin. 



Davidoff has pointed to the existence of structural similarities 

 between the hip-girdles of Chimeera and Cerutodus (7, pp. 142-3); 

 and if the magnificent array of structural affinities between the two, 

 so successfully demonstrated by Huxley (19), have the weight which 

 he assigns to them, I think it more than probable, if, as I have 

 suggested, the basal mesomere of Ceratodus is a derivative of the 

 metapterygium, that the paired fins of the Dipnoi may have arisen, 

 side by side with those of the Plagiostomes, from some such form 

 as is to-day represented by Chimeera — the fusion of the rays to form 

 the mesopterygium having gone on independently, the intercalation 

 of that structure between the applied bases of the pro- and meta- 

 pterygia, so characteristic of the Plagiostomes, having been a com- 

 paratively late process. 



1 The free ray represented at * in fig. 10 has been described by Davidoflf {op. 

 cit. p. 471). Tlie .spur-like outgrowth of the same, which I think may not im- 

 probably represent the coalesced vestige of a second similar one, was not 

 present in his specimen. 



