XIII 



PHYLUM CHORDATA 



159 



the centra are strengthened by radiating or concentric lamellae of 

 bone ; or they may be completely ossified. They are deeply 

 amphicoelous, the remains of the 

 notochord persisting in the large 

 inter-central spaces. Intercalary 

 pieces (Fig. 787, Ic.) are interposed 

 between both superior and inferior 

 arches. In the Rays (Fig. 788) the 

 anterior part of the spinal column 

 becomes converted into a continu- 

 ous solid cartilaginous and bony 

 mass the anterior vertebral plate 

 (a.v.p.) As in Fishes in general, 

 two regions are distinguishable in 

 the spinal column the prce-caudal 

 and the caudal, the latter being 

 distinguished by the possession of 

 inferior or haemal arches. In the 

 prae-caudal region short ribs may 

 be developed, but these are some- 

 times rudimentary or entirely ab- 

 sent. In the Sharks pterygio- 

 phores, sometimes jointed, fused 

 at their bases with the haemal 

 spines, support the ventral lobe 

 of the caudal fin, and the dorsal 

 lobe of the same fin is supported by a series of pterygiophores 

 resembling produced neural spines, but only secondarily related 

 to the spinal column and sometimes also divided by joints. The 

 dorsal and ventral fins are' sometimes supported by similar ptery- 

 giophores ; but in 



Ob Jc many cases the car- 



tilaginous supports 

 of these fins consist, 

 in whole or in part, 

 of expanded plates 

 of cartilage. 



The skull is an 

 undivided mass of 

 cartilage, hardened, 

 in many cases, by 

 deposition of osse- 

 ous matter, but 

 not containing any 



separate bony elements. It consists of a cartilaginous case for the 

 protection of the brain and the organs of special sense. The struc- 

 ture of this cartilaginous brain-case as it occurs in the Dog-fish has 



FIG. 786. Dermal denticles of Centro- 

 phorus calceus, slightly magni- 

 fied. (From Gegenbaur's Comparative 

 Anatomy.) 



\VK 



Fia. 787. Portion -of the spinal column of Scymnus. Ic. 

 intercalary cartilages ; Ob, neural arches ; TFA', centra. 

 (From Wiedersheim.) 



