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Show 1881.] OF THE PAIRED FINS OF ELASMOBRANCHS. 659 of Ceratodus from the typical Selachian type of pectoral fin. His own statement on this subject is as follows1:- " O n further inquiry into the more distant relations of the Cera-todus- limb, we may perhaps be justified in recognizing in it a modification of the typical form of the Selachian pectoral fin. Leaving aside the usual treble division of the carpal cartilage (which, indeed, is sometimes simple), we find that this shovel-like carpal forms the base for a great number of phalanges, which are arranged in more or less regular transverse rows (zones) and in longitudinal rows (series). The number of phalanges of the zones and series varies according to the species and the form of the fin; in Cestracion philippi the greater number of phalanges is found in the proximal zones and middle series, all the phalanges decreasing in size from the base of the fin towards the margins. In a Selachian with a long, pointed, scythe-shaped pectoral fin, like that of Ceratodus, we may, from analogy, presume that the arrangement of the cartilages might be somewhat like that shown in the accompanying diagram, which I have divided into nine zones and fifteen series. " When we now detach the outermost phalanx from each side of the first horizontal zone, and with it the other phalanges of the same series, when we allow the remaining phalanges of this zone to coalesce into one piece (as, in nature, we find coalesced the carpals of Ceratodus and many phalanges in Selachian fins), and when we repeat this same process with the following zones and outer series, we arrive at an arrangement identical with what we actually find in Ceratodus." While the researches of Thacker and Mivart are strongly confirmatory of the view at which I had arrived with reference to the nature of the paired fins, other hypotheses as to the nature of the skeleton of the fins have been enunciated, both before and after the publication of m y memoir, which are either directly or indirectly opposed to m y view. Huxley in his memoir on Ceratodus, which throws light on so many important morphological problems, has dealt with the nature of paired fins 2. He holds, in accordance with a view previously adopted by Gegenbaur, that the limb of Ceratodus " presents us with the nearest known approximation to the fundamental form of vertebrate limb or archipterygium," and is of opinion that in a still more archaic fish than Ceratodus the skeleton of the fin " would be made up of homologous segments, which might be termed pteromeres, each of which would consist of a mesomere with a preaxial and a postaxial para-mere." He considers that the pectoral fins of Elasmobranchii, more especially the fin of Notidanas, which he holds to be the most primitive form of Elasmobranch fin, " results in the simplest possible manner from the shortening of the axis of such a fin-skeleton as that of Ceratodus, and the coalescence of some of its elements." Huxley 1 Loc. cit. p. 534. 2 T. H. Huxley, " O n Ceratodus Fosteri, with some Observations on the Classification of Fishes," Proc. Zool. Soc. 1876. |