240 



VERTEBRATE SKELETON 



pi.r 



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The fin skeletons of some fossil Ganoids (fig. 258) is interesting as suggesting 

 the way in which the podia of Tetrapoda have been evolved (p. 245). 



Teleosts (fig. 250) have few radials (rarely more than five, some- 

 times but two or three) in the pectoral fin. These may be elongate 

 (Physostomes) or small plate-like particles, articulating or even 

 uniting with the pectoral girdle between scapular 

 and coracoid parts, the hinge of the fin lying 

 between them and the actinotrichia. Often there 

 are small cartilages between radials and actino- 

 trichia. Many Acanthopterygii have an ossified 

 spine at the anterior margin of the fin. The 

 ventral fins of Teleosts usually lack radials; when 

 present (some Physoclists) they are small. 



DIPNOI have the pectoral girdle less special- 

 ized than in Teleosts. The development is imper- 

 fectly known, so there is uncertainty as to the 

 elements. The cartilage stage is much like that 

 of normal Teleosts, the cartilage halves uniting in 

 the median line. There is a single ossification in 

 the glenoid region of Protopterus and Lepidosiren, 

 two in Ceratodus, scapular and coracoid in position, 

 but usually called cleithrum and clavicle, although 

 it is not known whether they be cartilage or mem- 

 brane in origin. As in Teleosts the girdle is con- 

 nected with the cranium by one {Ceratodus) or 

 two supracleithra {Protopterus, Lepidosiren). 



There is a true pelvic girdle, continuous from 

 side to side, with a prepubic process on either half 

 and a slender epipubic bar extends forwards from 

 the ischio-pubis. The pelvis arises from paired 

 cartilages. 



The three existing Dipnoan genera differ con- 

 siderably in fin structure. Ceratodus, the more 

 primitive, has pectorals and ventrals nearly equal, and the skeletons 

 of the two are more ahke than is usual in fishes. Each has a jointed 

 axis, the joints becoming smaller distally. The basal joints lack 

 radials, but all others have them on either side, the fin being biserial 

 (fig. 253). The radials support actinotrichia which extend nearly to 

 the margin of the fin. The pectoral radials are more numerous on 



Fig. 253. — Pelvic 

 fin and part of girdle 

 of Ceratodus (David- 

 oflf). 



