FINS OF ELASMOBEANCHS. 473 



Callorhynchus (Plate LXXIX. fig. 2), or the ventral oi Polyodon, the preaxial primitive 

 radials laid the foundation of the propterygial part of the fin, and the postaxial radials 

 of its metapterygial part. 



The same analogies apply to the ventral fin, the continuous basal part of which is 

 evidently the homotype of the pectoral metapterygium, while the preaxial radial or 

 radials serially repeat the propterygium, as is most plainly shown by the limbs of 

 Squatina. 



The radials, by increase, approximation, and coalescence, may be conceived as pro- 

 ducing the form of pectoral found in the Rays, and by further coalescence (through 

 SquatinaASke forms) that found in the Sharks and in most other fishes. 



According to this view, the fin of Ceratodus cannot (as supposed by Gunther and 

 Huxley) represent the archipterygium, and, far from being a primitive form ^ whence 

 the piscine limb may have been derived by a progressive shortening (as suggested by 

 Professor Huxley -), is, on the contrary, a very special and peculiar structure, which is 

 carried to a still more abnormal development in Lepidosiren, by progressive elongation 

 and by atrophy of the postaxial radials. And this relationship is surely what might 

 have been expected a priori. Surely, on the evolution theory, air-breathing vertebrates 

 were later developments, and such a structure as the limb of Ceratodus must have 

 long postdated that of the limbs of primeval Elasmobranchs, if not that of the earliest 

 Teleosteans also. A fortiori, then, Professor Gegenbaur's view, that the cheiroptery- 

 gium is due to a further continuation of that process by which the Elasmobranch fin 

 has, in his opinion, been formed from a Ceratodus-lUie limb, is quite fallacious^. 



Much more probable is the hypothesis of Professor Huxley, that the cheiroptery- 

 gium, " as an organ of support and locomotion," requiring " length, strength, and 



' Both Dr. Gunther and Professor Gegenbaur have suggested that the Ceratodas-]iaib may have resulted in 

 different ways from the coalescence of a longitudinal series of parts which, according to Professor Gegenbaur, 

 may have been like branchial rays. See Phil. Trans, vol. cLxi. p. 534, and ' Untersuchungen,' Heft 3, p. 181, 

 note. 



' Professor Huxley says : — " The most highly specialized forms of ichthyopterygium result from the shorten- 

 ing of the skeleton of the fin, the approximation of its distal elements to the shoulder-girdle, and the multi- 

 plication of its rays." — P. Z. S. 1870, p. 56. 



= i>r. Giinther says (l. c. pp. 532, 533^, as to the second cartilage of Ceratodus : — " Although externally it 

 appears as a single, flat, broad, short piece, unevennesses of its surface indicate that several priraarj' pieces are 

 coalesced in it. I am confirmed in this view by a horizontal section in which the lines of the former divisions 

 are preserved in the shape of tracts of a white connective tissue." Professor Huxley, iu his specimen, found 

 '■ no trace of such divisions," though examined in a microscopic section. Mr. Thacher observes, as to this 

 divergence (1. c. p. 300) : — " The fact that Huxley could find no sign of division in his specimens seems of 

 little weight in view of the complete fusion which we know takes place here and there in median fins." 



I may add that I have carefully examined the specimen described by Dr. Gunther, and I am perfectly con- 

 vinced of the coiTectuess of his observation. 



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