SKELETON 



113 



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 attachment to the bar, while the need for a similar 



attachment for the dorsal muscle 



results in an extension of the bar 



dorsally above the articulation of 



the limb, thus producing the typ>- 



ical girdle. The derivation of the fin 



of any fish from that of Cladoselache 



is easily imagined, but no satisfactory 



comparison of the fin with the leg 



has yet been made. 



In the skeleton of the 

 paired appendages the internal 

 supports or girdles and the 

 skeleton of the free appendage 

 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 articula- 

 tion of the limb. The girdles, 

 as well as the skeleton of the 

 free appendage, are always laid 

 down in cartilage, and in the 

 free appendage, aside from the 

 actinotrichia, no parts of other 

 than cartilaginous origin usually 

 occur. In the girdles mem- 

 brane bones may be added as 

 will appear below. 



In its typical state each 

 girdle consists of three ele- 

 ments, one dorsal and two 

 ventral, all meeting at the 

 point of attachment of the free appendage, and 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. 



Tig. 118. 



-Ventral surface of 

 after Jaeckel. 



Cladoselache, 



