xx] 



VERTEBRAL COLUMN 



459 



other : the upper set correspond to the ribs of Elasmobranchs, the 

 lower to the ribs of other fish and to the haemal arches of Elasmo- 

 branchs. There are usually twice as many neural arches as there 

 are centra, and every alternate one is small and does not meet its 

 fellow, and hence is called an intercalary piece: the haemal 

 arches are generally as numerous as the centra but occasionally 

 there may be ventral intercalary pieces. The cranium, visceral 

 arches and centra are all strengthened by a calcareous deposit 

 in the ground substance of the cartilage. This calcified cartilage 



4... 





Fio. 224. Dorsolateral view of the pectoral girdle and fins of a Dog-fish, 

 Scyllium canicula, xf From. Reynolds. The gaps between the radialia 

 are blackened. 



Hollow in the midventral part of the pectoral girdle which supports the 

 pericardium. 2. Dorsal (scapular portion) of pectoral girdle. 3. Meta- 

 pterygium. 4. Mesopterygium. 5. Propterygium. 6. Propterygial 

 radial. 7. Mesopterygial radial. 8. Metapterygial radial. 9. Out- 

 line of the distal part of the fin which is supported by horny fin-rays. 



is to be carefully distinguished from true bone, which is entirely 

 absent in Elasmobranchs. 



The primitive tail fin of Vertebrata, as we have seen, is a fringe 

 surrounding the end of the tail. Only a small and narrow remnant 

 of this persists in Elasmobranchs, the whip-like end of the tail being 

 bent up ; beneath it there is a well-marked fin, and this together 

 with the remains of the primitive caudal fin constitute a secondary 



