730 DEVELOPMENT OF THE SKELETON 



/ 



My observations on the development of the skeleton of the 

 fins certainly do not of themselves demonstrate that the paired 

 fins are remnants of a once continuous lateral fin ; but they sup- 

 port this view in that they shew the primitive skeleton of the 

 fins to have exactly the character which might have been an- 

 ticipated if the paired fins had originated from a continuous 

 lateral fin. The longitudinal bar of the paired fins is believed 

 by both Thacker and Mivart to be due to the coalescence of the 

 bases of the primitively independent rays of which they believe 

 the fin to have been originally composed. This view is probable 

 enough in itself, and is rendered more so by the fact, pointed 

 out by Mivart, that a longitudinal bar supporting the cartilagin- 

 ous rays of unpaired fins is occasionally formed ; but there is no 

 trace in the embryo Scylliums of the bar in question being 

 formed by the coalescence of rays, though the fact of its being 

 perfectly continuous with the bases of the fin-rays is somewhat 

 in favour of such coalescence. 



Thacker and Mivart both hold that the pectoral and pelvic 

 girdles are developed by ventral and dorsal growths of the ante- 

 rior 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 Thacker and Mivart as to the genesis of the 

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

 ject, they are distinctly favourable to those theories. 



The mai-n results of my observations appear to me to be 

 decidedly adverse to the views recently put forward on the struc- 

 ture 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 multisegmented 

 axis with numerous lateral rays. 



