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Show 6 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, what I imagine may represent the cartilage in question, and that, here the case, in irregular fins. Be it, as it there exists, what it may, its characters in the fin figured by m e are still further noteworthy. The entire fin-skeleton (fig. 1), with the exception of this bar and the proximal mesomere, is very slender and leaf-like ; the two elements just named (which, be it remembered, are in direct connexion) are relatively massive and much thicker and more powerful than the rest. The bar r, instead of being ellipsoidal in transverse section, as is invariably the case with even the most powerful parameres, is expanded along its free border in a manner strikingly suggestive of the metapterygium as it exists in many Elasmobranchs. It is segmented into a main piece and two small terminal ones, and appears, at first sight, to represent an element of greater importance than an ordinary ray. The fact that this new element appears in " irregular" fins, taken in conjunction with the fact that no such structure has hitherto been recorded for a " regular " fin, appears at first sight to detract from its novelty. Before proceeding further, therefore, three questions must be met:-1. H o w far is the fin under discussion abnormal? 2. Can the existence of the new element be demonstrated for a more normal fin ?, and 3. If so, under what structural conditions does it exist 1 Giinther, in his original description of the Ceratodus fin, described (14, p. 532) certain " slight irregularities" in the distribution of the rays. Huxley (19, p. 47), commenting upon these, remarks that they are "in respect of the median pieces .... constant peculiarities of no small importance." Davidoff (7, p. 126) describes the stem of the pelvic fin as consisting of a row of pieces " deren Zahl bei den verschiedenen Individuen betrachtlich variirt; " he adds-" nirgends fand ich ein so unregelmassiges Verhaltniss derselben zu einander, wie es Giinther auf seiner Figur abbildet." Other writers have observed this irregularity, and the last of them (Schneider) has formulated the distribution of the parameres of both fins. H e states (23, pp. 521-22), " bei der Brustnosse sitzt dorsalwarts am zweiten bis elften Gliede des Hauptstrahls, und zwar an der distalen Gelenk-flache, je ein Seitenstrahl. Ventralwarts sitzen am zweiten Gliede des Hauptstrahls hinter einander fiinf Seitenstrahlen, am dritten und vierten Gliede je zwei, an den folgenden einer. Bei der Bauchflosse tragen die Glieder des Hauptstrahls ventralwarts je einen Seitenstrahl, dorsalwarts je zwei Seitenstrahlen." I have taken some pains to test the reliability of this very definite statement, and am in a position to assert with equal assurance that the only constant character as yet recognized is the attachment of one ray to the preaxial border of each pectoral mesomere (cf. figs. 5 & 6). Even in so modified a fin as that of fig. 5, where several of the parameres are branched and two are directly confluent, this rule holds; and in no regular pectoral fin yet examined has an exception to it been found. I give below a table of average distribution of the parameres of those segments dealt with by Schneider, calculated out from observations made upon eight pectoral and ten pelvic fins. |