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Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 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 aud forwards, intercepts the proximal ends of two or three of the following rays, and comes into connexion with the proximal segment. The difficulty which arises out of this apparently natural interpretation of the parts of the skeleton of the fin of Chimeera 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 Chimeera, on the interpretation just given, indicates. If, following the analogy of Notidanus, we consider Mt in Chimeera 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 preserved. 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 fish 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 praeaxial 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. Gunther and Professor Gegenbaur go a step further, and suggest that even this archipterygium may be the secondary product 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 Umb. In this case, * ' Untersuchungen,' Heft ii. " Brustflosse der Fische," 1865, |