56 PROF. T. H. HUXLEY ON CERATODUS FORSTERI, [Jan. 4, 



third, and second digits represent so many prseaxial 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 poUex 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 is 

 the result of further progress, in the same direction, of the meta- 

 morphosis 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 chiropterygium, 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 approxi- 

 mation 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 ; {'A) the 

 second distal carpal and the index. The postaxial parameres are : — 

 (1) the ulna, the uluare, the fifth distal carpal, and the digitus mini- 

 mus; (2) tbe fourth carpal and the annularis. 



In fig. 1 1 the skeleton of tbe pectoral fin of Cestracion is repre- 

 sented side by side with the skeleton of the fore limbs of Meno- 

 branchus. Ichthyosaurus, Plesiosaurus, and Geclo ; 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. 



