130 THE VERTEBRATE SKELETON. 



type of fin, when it is once reached it is easy to derive all the 

 other types from it. The fins of the other living Dipnoi, 

 Protopterus and Lepidosiren are simply archipterygia from 

 which the radialia have almost or completely disappeared, 

 leaving only the segmented axes. Archipterygia too are found 

 in the pectoral fins of the Ichthyotomi, but the postaxial 

 radialia are much reduced. 



The ichthyopterygium, or type of fin, characteristic of 

 many modern Elasmobranchs such as Scyllium, may have been 

 derived from the archipterygium by the gradual reduction of 

 the rays on the postaxial side of the axis and their condensation 

 on the pre-axial side. The Ichthyotomi such as Xenacanthus 

 show one stage in the reduction of the post-axial rays, and a 

 further stage is seen in the Notidanidae and some other sharks 

 like Scymnus and Acanthias, in which a few postaxial rays 

 still remain. The condensation of the pre-axial rays when 

 further continued leads to one of the rays getting an attach- 

 ment to the girdle. Thus the fin comes to articulate with the 

 girdle by two basalia or basal pieces ; a third attachment is 

 formed in the same way and the three basalia are called re- 

 spectively pro-, meso-, and meta-pterygia. By some authors the 

 meta-pterygium and by others the meso-pterygium is regarded 

 as homologous with the axis of the archipterygium. 



The pectoral fins of Elasmobranchs vary very much in 

 their mode of attachment. In some of the sharks, including 

 the Notidanidae and Scy Ilium, all three basalia articulate with 

 the pectoral girdle, while in others such as Cestracion the 

 meta-pterygium is excluded. In Rays the pro-pterygium and 

 the meta-pterygium are long and narrow and diverge much 

 from one another; other basalia work their way in between 

 the meso-pterygium and meta-pterygium, and come to arti- 

 culate with the pectoral girdle. Sometimes they fuse and 

 form a second meso-pterygium. The radialia are greatly 

 elongated and are segmented. 



In Chimaera all three basalia are present, but the meso- 



