FINS OF ELASMOBRANCHS. 473 
Callorhynchus (Plate LX XIX. fig. 2), or the ventral of 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 
Squatina-like forms) that found in the Sharks and in most other fishes. 
According to this view, the fin of Ceratodus cannot (as supposed by Giinther 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 &@ priori. Surely, on the evolution theory, air-breathing vertebrates 
were later developments, and such a structure as the limb of Ceratoduws 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-like 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 
1 Both Dr. Giinther and Professor Gegenbaur have suggested that the Ceratodus-limb 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. 1876, p. 56. 
% Dr. Giinther says (J.c. pp. 532, 533), as to the second cartilage of Ceratodus :—‘ Although externally it 
appears as a single, flat, broad, short piece, uneyennesses of its surface indicate that several primary 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, in his specimen, found 
“no trace of such divisions,” though examined in a microscopic section. Mr. Thacher observes, as to this 
divergence (J. 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 haye carefully examined the specimen described by Dr. Giinther, and I am perfectly con- 
vinced of the correctness of his observation. 
382 
