104 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
basalia, the basalia of the two sides being unconnected with each other. Greater 
growth of the basalia would result in some of them meeting and fusing in the mid- 
dle line, thus forming a bar across the ventral side of the body, giving additional 
support to the fin. . Then to compensate for the rigidity, the basals become jointed 
on either side, leaving the medial bar 
with an articular surface on either side 
for the reduced basalia. The ventral 
muscles of the fin would find firm at- 
tachment to the bar, while the need for 
a similar attachment for the dorsal 
results in an extension of the bar dor- 
sally above the articulation of the 
limb, thus producing the typical girdle. 
The derivation of the fin of any fish 
from that of Cladoselache is easily 
imagined, but no satisfactory compari- 
son of the fin with the leg has yet been 
made. 
In the appendicular skeleton 
the internal supports or girdles 
and the skeleton of the free ap- 
pendage are to be recognized. 
Each girdle is an inverted arch 
crossing the ventral side of the 
body and extending up on either 
side above the articulation of 
the limb. The girdles, as well 
as the skeleton of the free ap- 
pendage, are always laid down 
in cartilage, and in the latter, 
aside from the actinotrichia, no 
parts of other than cartilaginous 
orginoccur. Inthe girdles mem 
brane bones may be added as 
Fic. 107.—Ventral surface of Cladoselache, will appear below. 
Bice Jaceeel In its typical state each girdle 
consists of three elements, one dorsal and two ventral, meeting at the 
point of attachment of the free appendage, all contributing to the 
socket (glenoid fossa, acetabulum) which receives the basal element of 
the skeleton of the limb. The limbs themselves are much alike in 
their general structure, as may be seen from the adjacent diagram. 
