SKELETON. 11g. 
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 
Fic. 123. Fic. 124. 
Fic. 123.—Pelvic fin and part of girdle of Ceratodus, after Davidoff.a, axial skeleton 
of fin; pil, iliac process; pim, processus impar; 7, radialia. 
Fic. 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. 
