7l8 DEVELOPMENT OF THE SKELETON 



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, pre- 

 sume that the arrangement of the cartilages might be somewhat 

 like that shewn 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 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 1 . 



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 ver- 

 tebrate 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 paramere." He considers that the 

 pectoral fins of Elasmobranchii, more especially the fin of Noti- 

 danus, which he holds to be the most primitive form of Elasmo- 

 branch fin, " results in the simplest possible manner from the 



1 T. H. Huxley, "On Ceratodus Fosteri, with some Observations on the Classifi- 

 cation of Fishes," Proc. Zool. Soc. 1876. 



