SKELETON. 



upper limb. Two proximal radials as that of the next limb segment, while the 

 skeleton of ankle and foot is derived from a corresponding number of distal radials 

 on the anterior side of the fin. The 'archipterygial theory' of Gegenbaur assumes 

 an appendage like that of Ceratodus (the * archipterygium ') as the type from 

 which all legs and other fins have been derived, by a 

 shortening of the axis and a loss of radials, chiefly on 

 the preaxial side. The two views are illustrated in the 

 adjacent sketches. No known facts of either embry- 

 ology or paleontology throw any certain light on the 

 matter. 



Cladoselache (fig. 107) and the lower ganoids 

 have what is apparently the most primitive type 

 of fin with a large number of basalia which 

 support a large number of radialia. From these, 

 as we go upward in the scale, there is a reduction 

 in the number of basalia, either by disappear- 

 ance or fusion, while the other parts are variously 

 modified. Thus in recent elasmobranchs the 

 characteristic number of basalia is three in the 

 pectoral, two in the pelvic fin. These are known, 

 from in front backward as the pro-, meso-, and 

 metapterygium, the middle one being absent 



FIG. 123. FIG. 124. 



FIG. 123. Pelvic fin and part of girdle of Ceratodus, after Davidoff.a, axial skeleton 

 of fin; pil, iliac process; pirn, processus impar; r, radialia. 



FIG. 124. Skeleton of pectoral fin of Xenacanthus, after Fritsch. 



from the hind limb. The numerous radials are jointed transversely 

 (fig. 109), permitting more flexibility, and these may be arranged 

 entirely on one side of the basalia (uniserial), or the metapterygium 

 may be prolonged as an axis, and while most of the radialia are 

 on the preaxial side, some may occur on the postaxial side (biserial) 

 as seen in the carboniferous shark, Xenacanthus (fig. 124). In the 

 recent species the skeleton of the fin is continued by actinotrichia. 



