THE ELASMOBRANCII FISHES 



79 



than those of the propterygium. In the rays these are of unusual interest. In 

 addition to those extending from the mesopterygial cartilage there are certain 

 other radials posterior to this cartilage, as we have said, which extend to the 

 girdle. Five such occur in Raja clavata, two or three in Squatina, a shark, and 

 a larger number in some other forms. The most interesting thing about these 

 extraradials is that in some of the rays they produce, as Howes (1890) has 

 shown, a fourth basal, the neopterygium {ne.p., fig. 83), indicated in Rhmo- 

 batis and well formed in Pteroplatea. 



In the rays the metapterygial radials are similar to those of the propteryg- 



Fig. 85. Pelvic fin and girdle, CMamydoselachus (A c?, B 5)- (From Goodey.) 

 For explanation see fig. 86. 



ium. In the sharks these normally are found in the adult on the anterior side 

 of the main metapterygial axis. Not infrequently, however, postaxial radials 

 are well developed in the embryo {Acanthias, Carcharias, and many others). 

 The significance of postaxial radials has been pointed out by investigators 

 seeking a solution of the early form of the paired limb. Those who hold that 

 the early type of limb was like that of the present-day lungfish, Ceratodus, 

 with a central axis and anterior and posterior rays, think that the postaxial 

 rays of sharks are remnants of a past condition. 



PECTORAL GIRDLE 



The right and left limbs of the girdle are incomplete dorsally except in the 

 rays, in which the upper tips may be firmly joined to the spinal column or to 

 each other. Each half of the girdle is composed of two pieces, one dorsal, the 

 scapula {sc, fig. 84), another ventral, the coracoid (co.). The scapula varies 



