172 



ZOOLOGY 



SECT. 



supported on a base of a substance somewhat resembling the bony 

 cement or crusta petrosa of the tooth. 



The skeleton is composed of cartilage, with, in many cases, 

 deposition of calcareous matter in special places notably in the 



jaws and the vertebral column. 

 The entire spinal column may be 

 nearly completely cartilaginous 

 (Hexanchus and Heptanckus), but 

 usually the centra are strengthened 

 by radiating or concentric lamellae 

 of calcified tissue ; or they may be 

 completely calcified. They are 

 deeply amphiccelous, the remains 

 of the notochord persisting in the 

 large inter-central spaces. Inter- 

 calary pieces (Fig. 834, /c.) are in- 

 terposed between both superior and 

 inferior arches. In the Rays (Fig. 

 835) the anterior part of the spinal 

 column becomes converted into a 

 continuous solid cartilaginous and 

 calcified mass the anterior verte- 

 bral plate (a. v.p.*). As in Fishes in 

 general, two regions are distinguish- 

 able in the spinal column the 

 pre-caudal and the caudal, the 

 latter being characterised by the 

 possession of inferior or haemal 

 arches. In the pre-caudal region short ribs may be developed, 

 but these are sometimes rudimentary or entirely absent. In the 

 Sharks pterygiophores, sometimes jointed, fused at their bases with 

 the haBmal spines, 

 support the ventral 

 lobe of the caudal 

 fin, and the dorsal 

 lobe of the same 

 fin is supported by 

 a series of pterygio- 

 phores resembling 

 produced neural 

 spines, but only 

 secondarily related 

 to the spinal 

 column, and some- 

 times also divided by joints. The dorsal and ventral fins are 

 sometimes supported by similar pterygiophores ; but in many 

 cases the cartilaginous supports of these fins consist, in whole or in 



FIG. 833. Dermal denticles of Centre - 

 calceus, slightly magni- 

 (From Gegenbaur's Comparative 

 Anatomy.) 



WK 



FIG. 834. Portion of the spinal column of Scymnus. Ic. 

 Ob, neu 



rtebrata.) 



intercalary cartilages ; Ob, neural arches ; WK, centra. 

 (From Wiedersheim's Vert 



