668 MR. F, M. BALFOUR ON THE SKELETON [June 1, 



skeleton ; and the observations have also, it appears to me, important 

 bearings on the theories of mypredecessors in this line of investigation. 



The most obvious of the positive conclusions is to the effect that 

 the embryonic skeleton of the paired fins consists of a series of parallel 

 rays similar to those of the unpaired fins. These rays support the 

 soft parts of the fin?, which have the form of a longitudinal ridge ; 

 and they are continuous at their base with a longitudinal bar. This 

 bar, from its position at the base of the fin, can clearly never have 

 been a median axis with the rays on both sides. It becomes the 

 basipterygium in the pelvic fin, which retains its embryonic struc- 

 ture much more completely than the pectoral fin ; and the metapte- 

 rygium in the pectoral fin. The metapterygiura of the pectoral fin is 

 thus clearly homologous with the basipterygium of the pelvic fin, 

 as originally supposed by Gegenbaur, and as has since been main- 

 tained by Mivart. The propterygium and mesopterygium are ob- 

 viously relatively unimportant parts of the skeleton as compared 

 with the metapterygium. 



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 support this 

 view in that they show the primitive skeleton of the fins to have 

 exactly the character which might have been anticipated if the paired 

 fins had originated from a continuous lateral fin. The longitudinal 

 bar of the paired fins is beheved 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 

 cartilaginous 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 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 tViat 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 subject, they are dis- 

 tinctly favourable 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 



