50 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jail. 4, 



edges of the fin ; and they differ only in the relative extent of the 

 central area, on which the fin-rays do not encroach. 



All the Chimseroids and Plagiostomes are eminently erossoptery- 

 gian so far as their fins are concerned ; and therefore we might 

 expect to find in the skeleton of the pectoral fins of these fishes a 

 modification of the skeleton of that of Ceratodus. But in most of 

 these fishes the skeleton of the fins has undergone such an amount 

 of metamorphosis that it is difficult to reduce it to the type of Cera- 

 todus. In Notidanus* , however, the skeleton of the pectoral fin 

 affords the key to the nature of this metamorphosis. Here (fig. 10) 

 there is an axial cartilage, the broad proximal end of which articulates 

 with the pectoral arch. Distally it diminishes in diameter, and ends 

 by a truncated face, with which another slender cylindrical cartilage, 

 also axial in position, is articulated. 



I take these two cartilages to represent the shrunken axis of the 

 fin of Ceratodus. The praeaxial basal angle of this axial mass is 

 occupied by a distinct cartilage. Whether this represents the proxi- 

 mal axial cartilage of Ceratodus, or whether it is the proximal prae- 

 axial ray, is not clear. 



The praeaxial edge of the principal axial cartilage, at some little 

 distance from this piece, presents a series of notches, with which are 

 articulated a corresponding number of praeaxial rays, while, as has 

 been already stated, a single ray is articulated to the base of the 

 terminal axial cartilage. The uppermost or proximal praeaxial ray is 

 two-jointed and broader than the others. On the postaxial side there 

 is a triangular cartilage (Mt), wide distally, very narrow proximally, 

 where it is connected with the proximal end of the axial cartilages. 

 Twelve postaxial rays are articulated with the wide distal edge of 

 this cartilage. I conceive that this triangular postaxial cartilage is 

 formed by the coalescence of the axial ends of the postaxial rays. 



The fin-skeleton of Notidanus thus results, in the simplest- possible 

 manner, from the shortening of the axis of such a fin-skeleton as that 

 of Ceratodus and the coalescence of some of its elements. 



In Cestracion (fig. 11) the same process is carried a step further. 

 Here there is a large cartilage (I) which articulates with the pectoral 

 arch by a concave facet, and corresponds with the proximal axial car- 

 tilage of Ceratodus and Notidanus. United by ligament with its 

 hinder concave margin is the triangular postaxial cartilage (Mt), 

 which obviously answers to the similarly shaped postaxial cartilage of 

 Notidanus, and which bears a series of postaxial rays, of which the 

 first is directed almost at right angles to the axis of the fin, as in 

 Ceratodus. The first praeaxial ray (R) is very large and bifurcates 

 distally. The ray which is attached to the distal angle of the axial 

 cartilage probably answers to the ray-like termination of the axial 

 skeleton in Notidanus ; but it is not distinguishable from the rest. 



In the Dogfish (Scy Ilium) (fig. 10) the further shortening of the 

 axis gives rise to still greater changes. The axial cartilage (meso- 



* The figure represents the fin of the foetal 'Notidanus id which I have already 

 referred. The figure of the fin of an adult of the same species given by Gegen- 

 baur (Untersilchungen, Heft ii.Taf ix. fig. 2) shows essentially the same structure. 



