1887.] PAIRED FINS OF CERATODT3S. 19 



mesopterygium, and by Gegeubaur (11) to represent the meta- 

 pterygium. The points of difference between the two sets of obser- 

 vers are so well known that recapitulation of them would be super- 

 fluous here. I incline most nearly to Huxley's view, and hold that 

 the axis represents mainly, if not wholly, the mesopterygium of 

 the Sharks ; but I regard its condition in the latter as typical of 

 its earlier and more primitive state. I moreover think it not 

 unlikely that the short-lobed fin of the Crossopterygidce will prove 

 to be of an earlier type of structure than that of the elongated one 

 of the living Dipnoi. Extended observations along the lines already 

 laid down by Traquair, in his Monograph on Tristichopterus (27), 

 are greatly to be desired. That the elements described by him are 

 probably, and that those described by Goldfuss (13) and Kner (20) 

 in Xenacanthus are certainly, homologous with those of the axis of 

 the Ceratodus fin, I fully "believe, and we have here the founda- 

 tion of a line of study which must soon yield fruitful results \ 

 Wiedersheim is the only worker who, to my knowledge, has 

 offered an opinion upon the last named. He says (29 p. 195), 

 speaking of the Ceratodus fin, "dass dieser Organisationsplan 

 der Brustflosse auch bei untergegangenen Fischgeschlechtern eine 

 Eolle gespielt haben muss, steht unzweifelhaft fest und ich mochte 

 dabei nur an den aus der Permformation stammenden Xenacanthus 

 decheni erinnern." 



There can no longer be much doubt that the confluence so 

 frequently seen between one or more rays and the mesopterygium of 

 the Elasmobranchs represents the last trace of the process by which 

 that structure is formed {cf. Dohrn, cited on p. 15). This granted, 

 it becomes a question as to how far the axis of the Ceratodus fin, 

 as here defined, represents a further extension of this fusion of pri- 

 marily parallel rays or an elongation of the mesopterygial plate, as it 

 exists in the Sharks. The fact that irregulaiity in distribution of the 

 parameres is generally accompanied by that of the segmentation of 

 the axis in Ceratodus {cf. especially figs. 1, 5, & 7), shows that 

 there is an intimate connexion between the two ; and this is the 

 more obvious on reflection that Davidoff has shown (7, p. 145) that 

 the segmentation of the axis does not stand in constant relationship 

 to the muscular attachments ^ It is moreover inconceivable, if 



regarder comme autant cle rayons membraux rest^s libres, et ils seraient les 

 homologues de ceux qui, par leiir asservation, donneut naissance aux membres 

 proprenient dits sur d'autres points du corps." 



1 I look with great satisfaction upon the work now being done in this direc- 

 tion by Smith Woodward {cf. P. Z. S. 1886). It is time that some such check 

 should be kept upon the deductions of the embryologist {cf. Baur, Morph. Jahrb. 

 yol. viii. p. 453). 



^ He writes, " Da wir auch an der Muskulatur eine den einzelnen Segmentea 

 der Stammreihe entsprechende Gliederung fanden, zo gewinnt diese Ansicht 

 an Wahrsheinlichkeit, obwohl immerhin noch einzuwenden ist, dass die 

 physiologische Bedeutung dieser Ghederung eine nur ausserst minimale sein 

 kann, dass ferner auch die Zwischensehnen der Stammuskulatur^ in gar keiner 

 naheren Beziehung zuden Segmenten der Stammenreihe sf.ehen." I could find 

 no difference between the muscles of the fin represented in fig. 6 and those 

 described by him. 



2* 



