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 follows ' : — 



" On further inquiry into the more distant relations of the Cei-a- 

 todus-\\mh, we may perhaps be justified in recognizing in it a modi- 

 fication 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 ia 

 Ceratodus.'^ 



While the researches of Thacker and Mivart are strongly confir- 

 matory 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 my memoir, which are either directly or indirectly 

 opposed to my view. 



Huxley in his memoir on Ceratodus, which throws light on so 

 many important morphological problems, has dealt with the nature 

 of paired fins '. 



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 homo- 

 logous 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 Notidanus, 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. 



^ T. H. Huxley, " On Ceratodus Fosferi, with some Observations on the 

 Classification of Fishes," Proc. Zool. See. 1876. 



