VERTEBRATE ANIMALS 227 



from the Devonian rocks, we find the pectoral and pelvic fins are 

 similar in shape to the dorsal fins of Scyllium, i.e. very broad at the 

 base and lying parallel with the long axis of the body. The radialia 

 of the pelvic fins are just simple rods of cartilage with their bases 

 embedded in the muscles of the body wall, but those of the pectoral 

 show signs of fusion at their bases. In some embryos a very similar 

 stage is passed through, and the two fins are even joined by a low 

 ridge of skin. The girdles are thought to have originated by a 

 further fusion of the somactidia, their sinking lower into the muscles 

 of the body wall and, finally, in order to form an efficient mechanical 

 support for the movement of the fin, extending markedly in a direc- 

 tion transverse to the long axis of the fish, with the consequent 

 development of an articulation between fin and support. Observa- 

 tions on the development of the paired fins, particularly with regard 

 to their muscle supply, also go to prove that they represent the 

 concentrations of fins that were originally joined to the body by a 

 very much longer base than at present. 



