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Show 12 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, establishment of the Elasmobranch fin, but in exactly the reverse order"1. The researches of Huxley and Balfour have proved that the propterygium of Gegenbaur (figs. 9 and 10, pt.) represents, throughout the Elasmobranch series, but one or more preaxial rays. It is the most variable of the three basal elements of the Shark's fin, and most observers are now agreed as to its morphological unimportance. The above-named writers are further at one in their estimate of the morphological value of the Elasmobranch meso-and metapterygia (ms., mt., figs. 9 and 10). That they disagree, however, upon at least one vital issue is well known, and the balance of opinion holds to-day that the solution of the ' archi-pterygium' question is to be sought in a reconciliation between their views. Huxley has described and figured (19, p. 48) the maximum development yet observed for the so-called propterygium of the Ceratodus pectoral fin. That structure cannot be definitely recognized in the pelvic fin. The determination of Huxley (19), Balfour (1), and v. Rautenfeld (22), which regards the axis of the Ceratodus fin as the mesopterygium, is too familiar to call for comment here. It must suffice to state that I accept it in the main, if not wholly, and assume for the present that the entire axis has the value which Huxley first assigned to it. It is at this point necessary to discuss, more fully than heretofore, the nature of the differences between the pectoral and pelvic fin-skeletons of Ceratodus. Schneider has asserted (23, p. 521) that " die Seitenstrahlen der dorsalen und ventralen Halfte der Flossen sind ungleich," also that the " Seitenstrahlen der dorsalen Halfte der einen Flosse entsprechen derjenigen der ventralen Halfte der anderen." There is an undoubted tendency towards the assumption of the condition which he" thus formulates for Ceratodus, and it seems to m e probable that a common determining cause may have led up to it and the condition realized in Protopterus (cf. Schneider, p. 524) ; but the definition no longer holds invariable for the former animal, in view of the facts thus far adduced. I have already stated that the presence of one preaxial paramere in connexion with each mesomere is a constant character of the Ceratodus pectoral fin, and I turn now to the distribution of the postaxial rays. I have given on p. 7 the average distribution for eight pectoral fins examined. The minimum observed was, taking the mesomeres in order of succession from within outwards, 3.1.2.1, the maximum 4.2.2.2. In no case have I observed five rays in attachment with the second mesomere, as stated by Schneider. Of the eight specimens examined, the second mesomeres of five bore each three rays; the third and fourth of seven each two ; and the fifth of six each one. It is thus certain that variation in the distribution of the postaxial parameres (cf. fig. 6) is, beyond doubt, far 1 Giinther originally advanced a somewhat similar opinion (14, p. 534), but he conceived of the process as having gone on along lines as yet incapable of support. |
