PECTORAL FINS OF POLYPTERUS. 



293 



it can be only a question of time for the ontogeny of tliis 

 important type of fins to be studied exhaustively, whilst embryo- 

 logical research cannot easily take up the question of the extent 

 of the amplitude of variations. 



The material examined consisted of 102 specimens of Polypterus 

 la])radii and Polypterus senegalus, varying in length from 12 

 to 43 cm. 



Specimens have been previously described by Gegenbaur*, 

 Klaatscht, Pollard +, and Budgett§. Unfortunately of larval 

 Polypterus only one specimen is known ; this was found and 

 described by Budgett. In his description, he says : " The shape 

 of the fin-blade is triangular, the metapterygfal border forming 

 with the distal border an acute angle, while the propterygial 

 border forms with the distal borcier an obtuse angle." None of 

 the specimens I examined had the larval shape ; the youngest, 

 12 cm. long, had already acquired the characteristic form of the 

 adult, the distal border of the mesopterygium being rounded 

 and produced near the metapterygium some distance beyond the 

 latter, thus forming a projection at this place. It seems probable 

 that the peculiar form of the pectoral fin, which has given 

 rise to so much controversy and which caused Gegenbaur and 

 Klaatsch to think it consisted of a stem carrying a biserial 

 arrangement of radials (a biserial archipterygium), is in a large 

 measure due to the partial ossification of the fin and the sub- 

 sequent unequal rate of growth of the bony and cartilaginous 

 pai'ts, the cartilaginous pai'ts growing more quickly. 



The radials showed variations both in number and form. : — 



I. Variations in Numiber. 



* Morph. .Tahrbuch, Bd. xxii. 1895, p. 119. 



t Pestschr. fiir C. Gegenbaur, Part I. 1896, p. 261. 



J Zoologische .Jahrbiicber, v. 1892, p. 387. 



§ Trans. Zool. See. London, vol. xvi. pt. vii., October 1902. 



