PINS OP ELASMOBEANCHS. 475 



and metapterygium, with their annexed cartilages, while all. or nearly all, the radials 

 of the opposite side became aborted. 



He has thus come to consider the metapterygium and the parts serially continuous 

 with it as the representatives of the main axis of the Ceratodus-limh ; and he has been 

 followed in this by A. Bunge \ who has brought forward interesting examples of what 

 he considers to be persistent traces of the originally biserial arrangement, persisting 

 towards the distal end of the line of the metapterygium. 



Professor Huxley has given ^ another interpretation. He considers the mesoptery- 

 gium and the parts continuous with it as the representatives of the limb-axis of Cera- 

 todus. This is an interpretation which logically follows from his conception of the 

 Elasmobranch pectoral as a drawing-in of the Ceratodus-ftn. He says ^ : — " In my 

 judgment the mesopterygium of Gegenbaur is the proximal piece of the axial skeleton, 

 which constantly retains its primary articulation with the pectoral arch." But, as we 

 have seen (Plate LXXIX. fig. 3), this element is certainly wanting, as a distinct part 

 in Chimcera and Callorhynclius, and most probably altogether from the skeleton of 

 CMloscyllium (Plate LXXVI. fig. 4). 



In Polypterus, again, its proximal part disappears, so that articulation with the 

 shoulder-girdle devolves entirely on the pro- and metapterygium. But whether 

 absent or not, such reduction in size and partial atrophy tells against its represent- 

 ing the true limb-axis of higher forms. Such axis must be rather represented by 

 the pro- or metapterygium ; but as to these a few observations will be more in place 

 later on. 



As has been said. Professor Huxley is inclined to derive the higher Vertebrate limb 

 from the Ceratodus type, and this by the atrophy of its proximal fore-and-aft radials 

 and the hypertrophy of its distal radials. Thus the main axis of the Ceratodus-\\mh 

 becomes the middle digit of the cheiropterygium, and the four other digits are the 

 terminations of the ultimate and penultimate radials of the two sides. 



His * special interpretation of the genesis of the hand from the Ceratodus-limb is 

 as follows : — " 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 prseaxial 

 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 fifth distal carpal, and the digitus minimus ; (2) the fourth carpal 

 and the annularis." 



' Jenaisch Zeifcsclirift, vol. vui. (1874), p. 293, plates 8 <fc 9. ' P. Z. S. 1876, p. 24. 



^ L. e. p. 55. ■• L. c. p. 66. 



