LIMBS 



123 



axis a number of segmented secondary rays arise on either side in 

 Ceratodus : these are not, however, strictly symmetrical (Fig. 101). 

 Beyond them horny rays are present, as in Elasmobranchs. A 

 proximal (basal) segment of the axis, which bears no rays, articu- 

 lates with the arch. In Protoptertis and Lepidosiren the fins, with 

 their skeleton, have undergone a marked reduction, so that little 

 more than the segmented axis remains. 



Thus the fins of Dipnoans differ from those of Elasmobranchs 

 (as well as of Ganoids and Teleosts) in being formed on a Mserial 

 type. 



Ganoidei. The skeleton of the fin is much simpler and the 



A 



FIG. 102. LEFT PECTORAL FIN OF A, Polyodon (Spatularia), AND B, Amia. 



I- IV, cartilaginous radii connected with the arch (S) ; a-g, radii which do not 

 reach the arch and are connected with the most posterior ray (IV in A, III 

 in B) : KS, bony rays. 



primary rays much fewer in number in Ganoids than in Elasmo- 

 branchs. 



In the pelvic fin of cartilaginous Ganoids more or fewer of the 

 radii unite together proximally to form a basale, which is perforated 

 by nerves, and from which a very primitive pelvic plate becomes 

 differentiated (p. 109, Fig. 87. B). It is important to bear in mind 

 that the distinction between an axis and secondary rays cannot 

 here, therefore, be strictly recognised, and the fin is thus more 

 primitive than in Elasmobranchs. 



The primitive relations have to a certain extent disappeared in 

 the pectoral fin of cartilaginous Ganoids, which, however, consists 

 of a varied number of rays. Of these, four reach the arch in 

 Polyodon (Fig. 102, A) and five in Acipenser. 



Fn the pectoral fin of Amia (Fig. 102, B) two large converging 

 marginal rays articulate with the shoulder-girdle, and only one 



