FINS OF ELASMOBRANCHS. 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-limb ; 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-fin. 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 LX XIX. fig. 3), this element is certainly wanting, as a distinct part 
in Chimera and Callorhynchus, and most probably altogether from the skeleton of 
Chiloscyllium (Plate LX XVI. 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-limb 
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 preaxial 
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 Zeitschrift, vol. viii. (1874), p. 293, plates 8 & 9. 2 Pp. Z. 8. 1876, p. 24. 
* L.c. p, 55. ‘ Lc. p. 56. 
