1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERl. 53 



to the postaxial edge of the second segment, and more and more of 

 the praeaxial rays to its praeaxial edge. At the same time the first 

 praeaxial ray, enlarging backwards and forwards, intercepts the 

 proximal ends of two or three of the following rays, and comes 

 into connexion with tlie proximal segment. 



The difficulty which arises out of this apparently natural interpre- 

 tation of the parts of the skeleton of the fin of ChimcBra consists in 

 this — that it leads to a doubt as to the true nature of the postaxial 

 cartilage {Mt) in Scyllium, and therefore in other Plagiostomes. For 

 this metapterygial cartilage cannot at the same time represent 

 coalesced postaxial rays, as the analogy of Notidanus would suggest, 

 and the second joint of the axial skeleton, as the analogy of Chimcera, 

 on the interpretation just given, indicates. If, following the analogy 

 of Notidanun, we consider Mt in Ghimcera to be formed of coalesced 

 postaxial fin-rays, then the structure will present no difficulty, but 

 will come very near that presented by the fin of Cestracion. The 

 study of the development of the parts can alone solve this problem ; 

 but I am inclined provisionally to adopt the latter hypothesis, 

 plausible as the former seems. 



Polypterus and Polyodon furnish the best connecting links 

 between the Plagiostome fin and that of the other Ganoidei and 

 the Teleostei. In Polypterus, the Scyllium type is essentially pre- 

 served. In Polyodon and all other Ganoids of which the fin- 

 structure is known, the type is essentially that of the Rays, in so far 

 as fin-rays enter into the glenoid articulation behind the proximal 

 median segment (I). These and many other special modifications 

 of the fish's fin have been carefully worked out by Gegenbaur*, to 

 whose excellent descriptions I have nothing to add. 



If the interpretation which I have here endeavoured to make good 

 is correct, it is clear that, as Gegenbaur has suggested, Ceratodus 

 presents us with the nearest known approximation to the fundamental 

 form of vertebrate limb, or archipterygium. But the asymmetry of 

 the skeleton of the fin of Ceratodus, and the differences between its 

 distal and its proximal portions, as well as the fact that the proximal 

 median segment has no rays, appear to indicate that the veritable 

 archipterygium has undergone a certain amount of modification even 

 in Ceratodus. Analogy leads to the suspicion that a still more 

 archaic fi^h than Ceratodus would have as many pairs of rays as 

 median pieces. In this condition the skeleton would be made up of 

 homologous segments, which might be termed pteromeres, each of 

 which would consist of a mesomere with a preeaxial and a postaxial 

 paramere. And as this is the actual state of a great portion of the 

 skeleton of the fin in Ceratodus, it may perhaps be permissible to 

 carry speculation as to the primitive condition of the vertebrate limb 

 thus far. Dr. Giiuther and Professor Gegenbaur go a step further, 

 and suggest that even this archipterygium may be the secondary pro- 

 duct of the coalescence of many longitudinal cartilaginous elements, 

 which are united by their bases, while they fray out, as it were, at 

 regular intervals towards the distal end of the limb. In this case, 

 * ' Unterduchungen,' Heft ii. " Brustflosse der Fische," 1865. 



