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Show 56 PROF. T. H. HUXLEY ON CERATODUS FORSTERI, [Jan. 4, third, and second digits represent so many praeaxial rays. The very serious objection that this hypothesis makes the radius and the radial digit postaxial, while, as a matter of fact, in every vertebrate animal it is praeaxial, is met by the assumption of a torsion of the humerus. But I must confess that I am wholly unable to satisfy myself of the existence of any torsion of the humerus capable of bringing about the effect attributed to it in any vertebrated animal; and, moreover, if such torsion has brought about the observed position of the manus and pes in the higher Vertebrata, any reversal of that torsion would destroy the homology of the pollex and the hallux-which is surely out of reach of doubt. I am disposed to think, though I am far from imagining that the hypothesis can at present be demonstrated, that the higher vertebrate limb has arisen from the archipterygium in another and simpler method. According to Gegenbaur's view, the higher vertebrate limb ia the result of further progress, in the same direction, of the metamorphosis which has given rise to the ichthyopterygium. But this appears to me to be highly improbable. The ichthyopterygium is specialized pari passu with the other peculiarities of piscine structure, and is not developed in the Dipnoi, which are the nearest allies of the Amphibia. Moreover the higher vertebrate limb, which may be termed the chir opt erygium, as an organ of support and prehension, requires length, strength, and mobility of its segments-conditions exactly the opposite of those which give the ichthyopterygium its special utility. Hence, as the most highly specialized forms of ichthyopterygium result from the shortening of the skeleton of the fin, the approximation of its distal elements to the shoulder-girdle, and the multipli-tion of its rays, we might expect that the chiropterygium would take its origin by the lengthening of the axial skeleton, accompanied by a removal of its distal elements further away from the shoulder-girdle, and by a diminution in the number of the rays. The parts which are traversed by a line drawn through the humerus, the intermedium, the centrale, the third distal carpal, and the third digit in the cheiropterygium may be regarded as so many mesomeres, representing the axis of the archipterygium. Two pairs of parameres are retained on each side. The praeaxial are :-(1) the radius, the radiale, the first distal carpal, and the pollex; (2) the second distal carpal and the index. The postaxial parameres are :- (1) the ulna, the ulnare, the filth distal carpal, and the digitus minimus; (2) the fourth carpal and the annularis. In fig. 11 the skeleton of the pectoral fin of Cestracion is represented side by side with the skeleton of the fore limbs of Menobranchus, Ichthyosaurus, Plesiosaurus, and Gecko ; and the shading of the different parts of the ichthyopterygium is repeated in what I suppose to be the homologous elements of the chiropterygium. In the case of Menobranchus, however, it is possible that the true pollex is suppressed, and that the actual radial digit represents the second of the pentadactyle limb, and therefore should have been left unshaded, |