236 



VERTEBRATE SKELETON 



tion as determined by other structures, but there is an advance over 

 Elasmobranchs, especially in the appearance of bone (both cartilage 

 and membrane) in the girdles, as well as in the modification and 

 reduction of the elements of the free appendages. 



GANOIDS. — In girdles, as in other structures, the Chondrostei 

 are apparently primitive in the absence of cartilage bones and in the 

 independence of the two halves of the girdles. The sturgeons (fig. 243) 

 show clearly the origin of the membrane bones, the only ossifications 

 of the girdles, which are more or less evidently repeated in all higher 

 fishes. The cartilage girdle persists as such through life. The 

 pectoral girdle has its halves connected by cartilage in the mid- 

 ventral line, the connexion being strengthened by the more external 

 membrane bones. The supracoracoid foramen is divided as in 

 skates (p. 232) and there is a large fenestra in the coracoid region 

 which is repeated in many Teleosts. The scapular region is strong, 

 and is continued dorsally by a suprascapular cartilage, connected 

 by ligament with the cranium. 



The membrane bones of the pectoral girdle show from the surface 

 and are clearly a part of the integument. The clavicle lies on 

 the ventral side of the coracoid region, those of the two sides meeting, 



in some species, in the middle 

 -^^ line; in others they are con- 

 nected by a dermal inter- 

 clavicle, which recalls the 

 reptilian episternum. This 

 ventral union strengthens a 

 girdle, weakened by the inde- 

 pendence of the coracoid 

 parts. The cleithrum meets 

 the posterior side of the 

 clavicle and extends up on 

 the outer posterior side of 

 the scapular cartilage. Be- 

 hind it are one or two small elongate postcleithra. The dorsal 

 end of scapula and suprascapular cartilage are overlaid by a series 

 of two supracleithra, the upper of which (posttemporal of authors) 

 enters the posterior dorsal wall of the cranium. In Cyprinoids and 

 Ganoids the supracleithra may also connect by ligament (sometimes 

 ossified) with the first vertebral centrum. 



Fig. 249. — Pectoral girdles of (.4 ) A mia (Good 

 rich, '09) and {B) Polyplerns (Gregory, '15). c 

 coracoid; cl, cleithrum; cv, clavicle; p, post 

 temporal (supracleithrum) ; r, radialia; .f, scap 

 ula; sc, supracleithra. 



