OF THE LAEVAL POLTPTEEUS. 329 



Klaatsch i and Gegenbanr -, though Gegenbaur appears now ^ to derive it directly 

 from forms with a very short axis. (2) The fin of Polypterus has been derived from a 

 uniserial fin of the type of Ileptanchus or, perhaps better, Chlamydoselache by the 

 shortening of the metapterygial border and the fusion of the bases of the radials to 

 form the expanded mesopterygium. 



Now the development of the fin might be expected to throw some light on this 

 question, and in the 30 mm. larva this is what we find : — The general shape of the fin 

 is quite different from that of the adult. The shape of the fin-blade is triangular, 

 the metapterygial border forming with the distal border an acute angle, while the 

 propterygial border forms with the distal border an obtuse angle (text-fig. 3. 2). 



The cartilaginous skeleton of the pectoral fin differs in many important points from 

 the skeleton of the fin of the adult Polypterus. The propterygium extending along 

 the preaxial border of the fin is stout but short, being scarcely more than one-third the 

 length of the metapterygium extending along the postaxial border, and being situated 

 almost at right angles to the latter (PI. XXXIII. fig. 8, Pro.). The angle between 

 these two rods is filled by a flattened blade of cartilage, which is, however, slightly 

 thickened where it is in contact with the metapterygium (PL XXXIII. fig. 8, Mes.). 

 Distally this flat blade or mesopterygium is split in a fan-like manner to form the 

 cartilaginous bases for the radials. The distal ends of these re-unite to form a 

 continuous border of cartilage. The mesopterygial blade is irregularly punctured for 

 the passage of blood-vessels (PL XXXIII. fig. 8, h.v.f.). On the free edge of the 

 metapterygium, at its distal end, is a slight flange of cartilage, seemingly forming 

 a rudimentary continuation round the distal end of the radial cartilages. 



In fact the pectoral fin of the larva is not strictly a crossopterygium, but is a 

 uniserial fin which can be directly compared with that of the Selachii. 



Tlie Pelvic Fin. — The cartilaginous skeleton of the very small pelvic fin consists of 

 a rod of cartilage unsegmented and slightly curved, approximating its fellow at the 

 anterior and posterior ends, but nowhere uniting with the latter (PL XXXIII. 

 fig. 9, Bus.). The anterior extremities taper slightly and diverge, there being no 

 indication of a distinct pelvic portion. Posteriorly these rods expand outwards and, 

 as they penetrate the body-wall, each becomes split into four or five finger-like rods, 

 some of which may re-unite distally. These form the cartilaginous bases of the radi.als 

 (PL XXXIII. fig. 9, r.). 



The development of the pelvic fin throws but little light on the morphology of the 

 fins of Polypterus, but seems to confirm to some extent the view here taken from 

 the development of the pectoral fin, that they have been derived from a uniserial type 

 by shortening of the postaxial border, rather than from a biserial type by shortening of 

 the median axis. 



' Festschr. von C. Gegenbfiur, part i. p. 261 (1896). 



- Jen. Zeit. vii. 1873. ^ Morph. Jahrb. sxii. p. 119. 



