50 



THE ELASMOBRANCH FISHES 



Skeleton of Paired Fins 



The skeleton of the pectoral fin (fig. 54) is fan-shaped; the proximal part con- 

 sists of three basal cartilages, propterygium, mesopterygium, and metaptery- 

 gium ; from the last two, nnmerous rows of radials radiate. 



The propterygium {pr.p.) in Heptanchus maculotus is a small nodule of 

 cartilage located upon the mesopterygium. It is followed by four or five large 



A. Female 



B. Male 



Fig. 55. The skeleton of the pelvic fin and girdle, Heptanchus maculatus. (Ruth Jeanette 

 Powell, del.) 



j3, beta cartilage ; h}'-, first and second connecting segments ; ha., basal or axial cartilage ; 

 ba.p., basipterygium ; pi., pelvic girdle ; ra., radials. 



radials, the first of which may fuse with the mesopterygium. The mesoptery- 

 gium (ms.p.) is a stout cartilage, from the enlarged distal end of which 

 extend ten or twelve rows of radials (ra.), depending upon the amount of 

 fusion which has taken place proximally. The most anterior row is composed 

 of large and irregular plates, but the remaining rows are broken up into small 

 segments. The metapterygium (mt.p.) is a large triangular cartilage, the base 

 of which points ventrally. It is segmented both j^roximally and distally and is 

 then continued into the most distal radial. From the metapterygium diverge 

 numerous rows of preaxial radials, in addition to which there are clearly 

 marked postaxial radials. 



