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Show 1887.] PAIRED FINS OF CERATODUS. 15 marks off the propterygium from the mesopterygium in the adult Cestracion. On a comparison of the two, I submit, with some degree of confidence, the opinion that the postaxial lobe of the second mesomere of Ceratodus (figs. 5 and 6, mt.) is the homologue of the Elasmobranch metapterygium. Comparison of that lobe and its attached rays (fig. 6) with the metapterygium of Cestracion and its rays (fig. 9) reveals a striking similarity, even in detail, between the two. Did the metapterygium of the Shark unite, as does its propterygium, with the axial plate ms., it would be difficult indeed to find a distinction between the first named and that which, in Ceratodus, I claim as its homologue. The homology which I here seek to establish bears out, with certain modifications, Balfour's view cited that " the fin of Ceratodus has been derived from a fin like that of the Elasmobranchs." That observer first recognized (1, p. 668) that the metapterygium (his basipterygium) is morphologically the most important and, phyloge-netically, the most primitive of the basal elements ; while he suspected (ibid.), but did not demonstrate the fact, that that structure is formed by the coalescence of rays. Huxley had already asserted this belief, in dealing with the metapterygium oi Notidanus (19, p. 50), which he regarded as being " formed by the coalescence of the axial ends of the postaxial rays" (presumably on the shortening of the fin axis). Dohrn has recently substantiated the deductions of Thacher, Mivart, and Balfour under this head, in having found that the metapterygium is (8, p. 174), in both pectoral and pelvic fins of the Shark, like the basal bar of the median fins, made up of " unpaare Knorpelstrahlen, die anfanglich oder jede Verbindung mit anderen Skelettelementen bleiben." He reiterates the statement on p. 182 in the words " was als Basipterygium beschrieben ist, stellt nur die verschmolzenen, wei sehr nah an einander leigenden, Basen der Flossenstrahlen dar und existirt nicht unabhangig von diesen." In face of the above facts, m y view demands that a primary distinction shall be demonstrated between the second pectoral mesomere in Ceratodus and that lobe which I hold to represent the metapterygium. In Giinther's original specimen (fig. 8) the said lobe was not represented in that which is now known to be its typical form, while the rays (r) which are usually attached thereto were for the most part independent. The proximal two of these appear to have been somewhat smaller than usual, but it is highly interesting to note that the two distal ones were uniting at their bases to form a plate-like structure (mt.) which showed no signs of confluence with the adjacent mesomere. This plate corresponds in its mode of origin with the metapterygium (basipterygium of Balfour), as defined by the above-named authors, and, in its relationships to the rest of the fin-skeleton, with the lobe now under consideration. I regard its condition as there represented to be indicative of the primary independence which m y interpretation necessitates. Gunther goes on to say (p. 532) that he found " lines of the former divisions" of the second mesomere of this specimen preserved, in the shape of tracts of white connective tissue. Huxley |