158 



COMPARATIVE ANATOMY 



anterior or marginal ray comes into close connection with the 

 cartilage of the primary fin-skeleton (Sturgeons) or entirely 

 replaces it (Amia). 



In the pelvic fin of cartilaginous Ganoids (Fig. 119) more or 

 fewer of the radii are connected proximally with a segmented 

 basale, which is perforated by nerves, and from which a very 

 primitive pelvic plate may in some cases become differentiated 

 (Fig. 104, B). It is important to bear in mind that the distinction 

 between an axis and secondary rays cannot, therefore, be strictly 

 recognised, as the basale corresponds to a number of fused radii, 

 and is perhaps not comparable to the metapterygium of Elasmo- 



A 



FIG. 120. LEFT PECTORAL FIN OF A, Polyodon, AND B, Amia. 



a ff, radii which do not reach the arch and are connected with the most 

 posterior ray (IV. in A, ///. in B) ; 7 IV, cartilaginous radii connected 

 with the arch (S') ; KS, bony dermal rays. 



branchs : but it is doubtful whether this character is primitive or 

 secondary. 



The primitive relations have to a certain extent disappeared in 

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

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

 in Polyodon (Fig. 120, A), and five in Acipenser. 



In the pectoral fin of Amia (Fig. 120, B) two large converging- 

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

 intermediate ray reaches the arch : this condition may be compared 

 with that seen in the highly-developed pectoral fin of Polypterus 

 (Fig. 121), which is flanked on either side by a strong, ossified ray, 

 between which is an intermediate region. The fin, therefore, 

 resembles that of an Elasinobranch Avith its propterygium, meso- 

 pterygium, and metapterygium. 1 



1 Even if it should be proved that the intermediate region (MS) no longer 

 arises in the embryo by a fusion of separate rays, it is possible that this was the 

 ease phylogenetically. 



