66 MR. F. M. BALFOUR ON THE SKELETON 



Thacher and Mivart both hold that the pectoral and 

 pelvic girdles are developed by ventral and dorsal grovrths 

 of the anterior end of the longitudinal bar supporting the 

 fin-rays. 



There is, so far as I see, no theoretical objection to be 

 taken to this view ; and the fact of the pectoral and pelvic 

 girdles originating continuously and long remaining united 

 with the longitudinal bars of their respective fins is in favour 

 of it rather than the reverse. The same may be said of the 

 fact that the first part of each girdle to be formed is that in 

 the neighbourhood of the longitudinal bar (basipterygium) of 

 the fin, the dorsal and ventral prolongations being subsequent 

 growths. 



On the whole my observations do not throw much light 

 on the theories of Thacher and Mivart as to the genesis of 

 the skeleton of the paired fin ; but, so far as they bear on 

 the subject, they are distinctly favorable to those theories. 



The main results of my observations appear to me to be 

 decidedly adverse to the views recently put forward on the 

 structure of the fin by Gegenbaur and Huxley, both of 

 whom, as stated above, consider the primitive type of fin to 

 be most nearly retained in Ceratodus, and to consist of a 

 central mulfciseg-mented axis with numerous lateral ravs. 



Gegenbaur derives the Elasmobranch pectoral fin from a 

 form which he calls the archi pterygium, nearly like that of 

 Ceratodus, with a median axis and two rows of rays — but 

 holds that in addition to the rays attached to the median 

 axis, which are alone found in Ceratodus, there were other 

 rays directly articulated to the shoulder-girdle. He considers 

 that in the Elasmobranch fin the majority of the lateral rays 

 on the posterior (or median according to his view of the 

 position of the limb) side have become aborted, and that the 

 central axis is represented by the metapterygium; while the 

 pro- and mesopterygium and their rays are, he believes, 

 derived from tbose rays of the archipterygium which 

 originally articulated directly with the shoulder-girdle. 



This view appears to me to be absolutely negatived by the 

 facts of development of the pectoral fin in Scyllium — not so 

 much because the pectoral fin in this form is necessarily to 

 be regarded as primitive, but because what Gegenbaur holds 

 to be the primitive axis of the biserial fin is demonstrated to 

 be really the base, and it is only in the adult that it is con- 

 ceivable that a second set of lateral rays could have existed 

 on the posterior side of the metapterygium. If Gegenbaur's 

 view were correct, we should expect to find in the embryo, if 

 anywhere, traces of the second set of lateral rays ; but the 



