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 

 orgin occur. In the girdles mem 

 brane bones may be added as 

 will appear below. 



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. 



FIG. 107. Ventral surface of Cladoselache, 

 after Jaeckel. 



