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Show 1887.] PAIRED FINS OF CERATODUS. 5 figure, and it had first to be ascertained from which side of the body it was derived. Its proximal mesomere (m.p., fig. 1) carries a large tubercle (tb.), which, as Schneider has lately pointed out, " bei der Brustflosse ventral, bei der Bauchflosse dorsal steht "-when the limb is in apposition with the body-wall. This process is, in all pelvic fins examined by me, somewhat crescent-shaped and outwardly directed, its inner face being excavated. In the fin under discussion its outer surface was flattened ; but as its inner one sloped obliquely outwards, I conclude that that fin was a right-sided one. It is represented in the figure as seen from the dorsal aspect. Its axis is for the most part unequally segmented and irregular, the proximal mesomere being the least modified portion thereof, as compared with the more normal fin. The second mesomere is greatly elongated, and it bears upon its postaxial border (left hand of the figure) a notched lobe, with which are connected five parameres. The two distal of these break up peripherally, and, on examining the individual specimen, it is hard to conjecture how far the lines of demarcation between the parameres and the lobe, and between it and the main piece of the axis, may represent the last traces of original lines of separation, or the lines of cleavage of a primarily continuous sheet. Preaxially, the second mesomere carries five parameres ; these are fairly uniformly set upon it, and the distal one of the series branches in a true dichotomy. Interposed between the free ends of the two proximal of these rays there is a smaller one (marked * in the figure), which I take to resemble those found by Davidoff (7, p. 127), occasionally lying free at the distal end of the fin. The rest of the skeleton is chiefly remarkable as concerns the axis ; this appears to be longitudinally cleft, and made up of a longer preaxial and a shorter postaxial piece, both of which are very irregularly segmented. All the parameres borne upon it, however, are simple unbranched rods, which differ from those more generally present only as regards their feeble segmentation. On examining the above-named fin with care, m y attention became arrested by the cartilage marked r in the figure, the characters and relations of which are altogether exceptional. Wiedersheim has called attention (30) to the fact that in Protopterus the basal segment of the axis may bear a lateral piece. To the consideration of this I shall return. In no regular Ceratodus fin (i. e. that bearing an equally segmented axis) yet described has there been found, postaxially, a cartilage like the above named, attached directly to the basal mesomere. That element is generally held to be destitute of rays. Gunther has figured (14, pl. 36. fig. 4) a pelvic fin of the right side, which bears lateral cartilages in the above-named region ; but I find no mention of the fact in his text. It is to me inexplicable for what reason he should have failed to describe so remarkable a feature. I shall return, in the sequel, to the discussion of this fin. Haswell has figured and described (15, figs. 5, 6, 7 x) 1 His fig. 2 is said to be a representation in outline of tbe pectoral fin, after Huxley. It is unfortunate that the boundary-line between the two basal mesomeres, indicated in the original, should have been omitted. |