78 



THE ELASMOBRANCH FISHES 



Heterodontus pMlippi, although this does not occur in Heterodontus francisci 

 (fig. 82). The mesopterygium is usually an independent piece, but it may be 

 masked by fusion as in Pristiophonis japonicus. A considerable change from 

 the embryonic plan of the basals is found in Scymniis lichia, in which only a 

 single basal cartilage is present. It is supposed that the missing basal carti- 

 lages have secondarily fused in the adult. 



The form of the fin in the rays differs greatly from that of a shark like Hep- 

 tanchus in that the basal cartilages are modified in keeping with, the dorso- 

 ventral depression and the great extent, anteroposteriorly, to which the fins 

 are expanded. The pectoral of Squatina, although shark-like in its articulation, 



-f.pt. 



A B 



rig. 84. Pectoral girdles. A. Heterodontus francisci. B. Torpedo. (Modified from Gegen- 

 baur.) 



CO., coracoid; f.pt., aperture for nerve and blood vessels to pectoral fin; sc, scapula. 



is like that of the rays in extent; it has, however, a much heavier mesopter- 

 ygium than have the rays. Usually the propterygium of the rays is divided 

 into a number of segments which extend forward to join the antorbital process 

 {Bhinohatis, fig. 83, pr.p.; Raja, Myliohatis) , or even to the tip of the ethmoid 

 region where the two from the opposite sides unite (Urolophus) . The mesop- 

 terygia of the rays are very variable. In some forms a mesopterygium is absent 

 whereupon the radials extend to the girdle. In other types there is a consider- 

 able mesopterygial plate, as in Rhinohatis {ms.p., fig. 83). In others still, a 

 second plate back of the mesopterj'^gium may be formed, as in Pteroplatia. The 

 metapterygium of the rays, like the propterygium, is greatly developed, pass- 

 ing backward to the region of the pelvic fin. 



The propterygial radials of the sharks are usually few in number. They may 

 form a single line of segments which may be of more or less regular plates, as 

 in Heptanchus or Heterodontus francisci (fig. 82) . In others the rows may be 

 more numerous. In the rays there are many rows of such propterygial radials, 

 some of which are made up of great numbers of segments (Rhinobatis, fig. 83) . 



The radials attached to the mesopterygium in the sharks are more numerous 



