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Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 47 the last. A more important point is that the second shows no trace of such divisions as those described by Dr. Gunther. To make sure of this I made a thin microscopic section of this cartilage on the right side, and thereby satisfied myself of the homogeneity of the cartilage of which it is composed. I find no true joint between the proximal median piece and the scapular arch, the connexion between the two being effected by a solid fibrous mass. Again, the "slight irregularities" in the distribution of the rays, in respect of the median pieces, of which Dr. Gunther speaks, are constant peculiarities of no small importance. This becomes obvious when the fin of Ceratodus is compared with that of other fishes. But before proceeding to this point I must make a few remarks on the normal and primitive position of the vertebrate limb, and on the changes from that normal position which take place in fishes on the one hand, and the higher Vertebrata on the other, as, for want of attending to this fundamental matter, grave errors have crept into the interpretation of the parts of the limbs of different vertebrates. The fins occupy the normal position in such a fish as Scyllium. When the axis of the body is horizontal the plane of the fin is also horizontal. Its upper surface is covered by a continuation of the dorsal integument, and its lower surface by that of the ventral side of the body. The distinctive spotting of the dorsal aspect in the Dogfish makes this very plain ; therefore, to adopt the nomenclature which I some time ago proposed, the convex thick edge of the fin is praeaxial, while its concavo-convex thin edge is postaxial, and its two aspects look respectively upwards and downwards. In Ceratodus, however, the fin has undergone a rotation upon its axis, in virtue of which its proper ventral surface looks more or less outwards, and its proper dorsal surface more or less inwards; and at the same time the praeaxial edge is turned upwards, while the postaxial edge is turned downwards. This is very marked when the fin is applied to the trunk ; but the primitive disposition of the surfaces and edges of the limb becomes obvious when the fin is made to stand out at right angles to the axis of the body. In Acipenser, as in any Teleostean fish, the rotation becomes still more marked; and divarication of the fin does not greatly diminish it, by reason of the articulation of the praeaxial fin-ray with the pectoral arch at a point higher than the proper glenoid cavity. When Ceratodus, or a Teleostean fish, is placed side by side with a Urodele Amphibian, such as Menobranchus, in which the fore limb naturally takes a position nearly parallel with the axis of the trunk, the manus being directed backwards, nothing is more natural than to imagine that the outer and inner aspects and the dorsal and ventral edges of the two correspond. But a very little consideration will show that nothing can be more erroneous ; for the outer surface of the Menobranchus limb is its proper dorsal aspect, and the inner surface is its ventral aspect, as will be seen the moment the limb is placed at right angles with the trunk. In fact, though in the amphibian, as in the fish, the limb |
