32 VERTEBRATE ZOOLOGY 



lates with the ventral fins. The skeleton of the ventral fin con- 

 sists of a long, more or less curved rod of cartilage called the 

 basipterygium, which extends straight back from the girdle. The 

 cartilaginous fin rays articulate with this and the girdle ; the distal 

 portion of the fin is supported by dermal horny rays. 



In the male the basipterygium is much extended by the addi- 

 tion of a second long cartilaginous rod at its posterior end, which 

 is grooved on its upper surface and forms the clasper. 



Exercise 39. Draw the ventral fin and girdle. 



The pectoral girdle is a U-shaped cartilaginous arch which lies 

 transversely across the ventral and on the lateral sides of the 

 body, immediately back of the gill arches. On each side of the 

 ventral portion of the girdle is a depression, the glenoid fossa, in 

 which the fin articulates, and which divides the girdle into a 

 dorsal or scapular and a ventral or coracoid portion. 



The pectoral fin articulates with the pectoral girdle by three 

 basal cartilages,— the propterygium, the mesopterygium, and the 

 metapterygium,— of which the last named is the hindermost. Along 

 the outer margin of these cartilages is a series of cartilaginous fin 

 rays, many of which are segmented ; these fin rays do not, how- 

 ever, support the entire fin but only its basal portion, the distal 

 half being supported by dermal horny rays. 



Exercise 40. Draw the pectoral girdle and fin, showing exactly the 

 outlines of all the cartilages. 



