SKELETON. 



175 



those parts which are shaded being those which persist in the 

 higher vertebrates. 



In the fin skeleton of the elasmobranchs two basals, an 

 anterior protopterygium and a posterior metapterygium, occur in 

 the ventral fin, while in the pectoral fin a third basal, the mesop- 

 terygium, is intercalated between the other two. These basals 

 support a richly developed radial system, the radialia being with 

 few exceptions developed on one side of the axis formed by the 

 basals. In the lower ganoids, on the other hand, the basals are 

 more numerous, and show the primitive conditions more clearly 

 than do the elasmobranchs. In both of these groups the rays 

 of dermal origin are well developed, and reach their extreme in 



FIG. 190. Typical pentadactyle limbs ; above fore limb, below hind limb. 

 c, centrale ; cp, carpus; f, femur; fe, fibulare ; Ji, fibula; h, humerus; i, interme- 

 dium ; me, metacarpals ; mf, metatarsals ; /', /", phalanges ; r, radius ; re, radiale ; 

 t, tarsus; te, tibiale; //, tibia; u, ulna; ue, ulnare ; I-V, digits; 1-5, carpales or 

 tarsales. 



teleosts in which the cartilage bones of the fin are greatly re- 

 duced. In the pectoral fin they are represented by (usually) 

 four bones (actinosts) which support the dermal rays, either 

 directly or by the intervention of cartilaginous radials. In the 

 dipnoi there is a well-developed and segmented axial skeleton 

 to the fin which may be without other skeletal parts {Protopterus, 

 Fig. 269), or which may bear biserial lateral branches which 

 connect with the dermal rays (Ceratodns, Fig. 185). 



Throughout the higher vertebrates, from the amphibia to 

 man, the same type of limb structure is everywhere found, that 



