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 tiie body-wall. This process is, in all 

 pelvic fins examined by me, somewhat crescent-shaped and out- 

 wardly 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 com- 

 pared 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 de- 

 marcation 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 imiformly set upon it, and the distal one of the series branches in 

 a true dichotomy. Interposed between the free ends of the two proxi- 

 mal of these rays there is a smaller one (marked * in the figure), 

 which I take to resemble those found by Davidoff (7, p. 127), occa- 

 sionally 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 difl^er from those more generally present only as regards their 

 feeble segmentation. 



On examining the above-named fin with care, my 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 tact that in Protopterus the basal seg- 

 ment 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 

 equajly segmented axis) yet described has there been found, post- 

 axially, a cartilage like the above named, attached directly to the 

 basal mesomere. That element is generally held to be destitute of 

 rays. Giinther has figured (14, pi. 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 inex- 

 plicable for what reason he shoidd 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 ') 



' His i3g. 2 is Scaid to be a representation in outline of the pectoral fin, after 

 Huxley. It is unfortunate that the bouudary-Iine between the two basal 

 mesomeres, indicated in the original, should have been omitted, 



