THE SKELETON IN FISHES. PAIRED FINS. 131 



pterygium is shifted and does not articulate with the pectoral 

 girdle J . 



In Acipenser and Polyodon the pectoral fin is built on the 

 same type as in Elasmobranchs, but becomes modified from the 

 fact that the pro-pterygium is replaced by dermal bone which 

 forms a large marginal ray. Extra meso-pterygia are formed 

 in the same way as in Rays. 



In Polypterus the pro- and meta-pterygia have ossified 

 while the meso-pterygium remains chiefly cartilaginous; the 

 fin-rays are also chiefly ossified. 



In Amia, Lepidosteus, and certain Teleosteans like Salmo, 

 not only the pro-pterygium but the meso-pterygium is almost 

 suppressed by the marginal ray. 



In the great majority of Teleosteans a still further stage is 

 reached, the endoskeletal elements, the basalia and radialia 

 are almost entirely suppressed and the fin comes to consist 

 entirely of ossified fin-rays of dermal origin. 



In some Teleosteans Exocaetus, a herring, and Dactylo- 

 pterus, a gurnard the pectoral fins are so enormously developed 

 that by means of them the fish is able to fly through the air 

 for considerable distances. The skeleton of these great fins is 

 almost entirely composed of dermal bone. 



PELVIC FIN. 



The pelvic fin is almost always further removed from the 

 archipterygial condition, and is in general more modified than is 

 the pectoral. Thus in the Ichthyotomi, while the pectoral fins 

 are archipterygia similar to those of Ceratodus, the pelvic fins 

 consist of an axis bearing rays on the postaxial side only, and 

 prolonged distally into a clasper. In Dipnoi however the 

 pelvic fins are very similar to the pectoral. In Elasmobranchs 

 the meso-pterygium is missing, the pro-pterygium is small or 

 absent, and the fin is mainly composed of the meta-pterygium 



1 Some of these views with regard to the homologies of the parts of 

 the fins are not accepted by all anatomists. 



92 



