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Show 1887.] PAIRED FINS OF CERATODUS. / Schneider Observed Schneider. Observed . Pectoral fin. Postaxial. Seg. ii. iii. iv. . 5 2 2 . 3-5 1-9 1-6 Pelvic fin. Postaxial. Seg. ii. . 1 . 2-1 v. 1 1-3 Preaxial. ii. 1 1 Preaxial. ii. 2 2-5 Further comment upon the pectoral member may be deferred until later. Concerning the ten pelvic fins examined by me, I may add that in eight the second mesomere bore preaxially two parameres (figs. 3 & 7); in a ninth three; in a tenth four. In most cases two postaxial rays were present (fig. 7). One fin, interesting beyond this, bore (fig. 2, right hand, as drawn) preaxially two rays, postaxially four, that being a precise reversal in duplicate of the condition observed by Schneider. In no case have I observed the distribution recorded by him. The parameres of all the fins alluded to were, for the most part, rod-like and segmented; but in not a few instances they were branched or otherwise modified (cf. figs. 1, 5, 7). Reflection upon the facts recorded concerning them, to say the least, shakes our trust in the supposed regularity of their distribution. That, however, can no longer be asserted, in view of the truly remarkable condition of one pair of fins, which belonged to a fish in all respects normal and healthy (fig. 2). Giinther first directed attention to the sickle-shaped contour of the Ceratodus fin, and all subsequent observers are agreed as to the asymmetry of its two lobes. Schneider states (23, p. 521):-"das zweite Glied des Hauptstrahls zerfallt durch eine Langsgrube in zwei Stiicke. Das eine Stuck behalt die Richtung des Hauptstrahls, das andere Stiick divergirt mit demselben und zwar bei der Biustflosse dorsalwarts, bei der Bauchflosse ventralwarts." And further, " Die Seitenstrahlen der dorsalen Hiilfte der einenFlosse entsprechen derjenigen der ventralen Halfte deranderen." A cursory glance at the pair of fins n ow under consideration (fig. 2 l) is sufficient to show how erroneous is this deduction. That Schneider has accurately represented the facts for the animals at his disposal, I have no doubt; but that his conclusions are incapable of a wider application is here proven. 1 I was at one time under the impression, from an examination of Davidoff's figures (7, pl. 9. figs. 6 & 7), that he had been dealing with a similar pair of fins ; but I a m no longer in doubt. His drawing of the fin-skeleton of fig. 7 is not in accord with the description given, as regards the pelvis and basal mesomere. He, moreover, states emphatically (p. 127), " die Zahl der ventralen resp. medialen Eeihe [referring to the parameres] entspricht genau der Zahl der Gliedstiicke des Stammes, wahrend diejenige der dorsalen resp. lateralen Eeihe fast genau u m das Doppelte grosser ist ? " |