OF THE PAIRED PINS OF ELASMOBRANCHS. 55 



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 Thacher and Mivart are strongly 

 confirmatory of the view at which I had arrived with refer- 

 ence 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 approximation to the fundamental form of verte- 

 brate limb or archipterygiura/^ 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 meso- 

 mere with a preaxial and a postaxial paramere." He con- 

 siders that the pectoral finn 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 does not enter into the 

 question of the origin of the skeleton of the pelvic fin of 

 Elasmobranchii. 



It will be seen that Huxley's idea of the primitive structure 

 of the archipterygium is not easily reconcilable with the 

 view that the paired fins are parts of a once continuous 

 lateral fin, in that the skeleton of such a lateral fin, if it 

 has existed, must necessarily have consisted of a series of 

 parallel rays. 



Gegenbaur^ has done much more than any other living 



^ T. H. Huxley, " Oa Ceratodus Fosieri, with some Observations on the 

 Classification of Eishes," 'Proc. Zool. Soc.,' 1876. 



■^ C. Gegenbaur, ' Untersuchungen z. vergleich. Anat. d. Wirbelthiere ' 

 (Leipzig, 1861-5) : erstes Heft, Carpus u. Tarsus ; zweites Heft, Brust 

 flosse d. Eische. 



" Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. 

 Hintergliedmaassen d. Selachier insbesondere," ' Jcnaische Zeitschrift,' vol. 

 v, 1870. 



" Ueb. d. Archipterygium," * Jenaische Zeitschrift,' vol. vii, 1873. 



" Zur Morphologic d. Gliedmaassen d. Wirbelthiere," ' Morphologisches 

 Jahrbuch,' vol. ii, 1876. 



