SKELETON. 



173 



median fin to be described later. From the more ventral folds 

 arose the ventral portion of the median fin, behind the vent ; 

 while the pectoral and ventral fins arose as differentiations 

 from the preanal region of the folds. In fact, several existing 

 elasmobranchs exhibit exactly this condition in their develop- 

 ment. 



In those parts of the fold where the fins are to form, rod- 

 like cartilage supports arose, possibly agreeing in number with 

 the myotomes concerned in the formation of the appendage. 

 At first these were separate and nearly equal in size, but later 





FIG. 187. Diagram of the origin of median and paired fins, from Wiedersheim. 

 A, with continuous fin folds; B, with differentiated fins. AF, anal fin; An, anus; 

 jRf, ventral fin ; BrF, pectoral fin ; Z>, dorsal folds ; FF, adipose dorsal ; RF, 

 dorsal fin ; S, lateral folds ; Sf, caudal fin. 



the basal portions became larger and separated from a more 

 distal (radial) part. Such a condition is seen in the extinct elas- 

 mobranch CladoselacJie (Fig. 188) ; but usually the basalia fuse 

 into a few larger elements, connecting the radialia together, and 

 giving stiffness to the whole fin. One of these enlarged basalia 

 acquired prominence over its fellows, and growing in toward the 

 median line fused with a similar ingrowth from the opposite side, 

 thus giving rise to the ventral portion of the girdle. But such 

 a bar would prove too rigid, and would prevent the fin from 

 moving freely, so there appeared a joint on either side, the distal 

 portion now articulating with the median or girdle region at a 

 place known as the glenoid fossa or acetabulum. The skeleton 



