62 VERTEBRATE ZOOLOGY 



two, the scapula and coracoid, articulate with the basals, the 

 former of these bones being dorsal to the latter. These are 

 cartilage bones and represent the pectoral girdle of the elasmo- 

 branch. In addition to these there are a number of membrane 

 bones which join the fin with the cranium. The cleithrum, or 

 so-called clavicle, is the largest of these and extends upward 

 from the anterior border of the scapula and coracoid. Its dorsal 

 end joins the supracleithrum, which in turn is joined with the 

 skull by the V-shaped posttemporal. 



Exercise 38. Draw the pectoral fin and girdle. 



The Ventral Fins. Each of these consists of about five radials. 

 At their proximal ends they articulate with a large plate which 

 represents the fused basals. These two plates lie alongside of each 

 other in the median area ; no pelvic girdle is present. 



Exercise 39. Draw the ventral fins. 



