SKELETON OF CHONDKOPTEKYGIANS. 



67 



centric or radiating lamellae of bone varies greatly in the 

 various genera, and according to the age of the individuals. 

 In the Eays all the vertebrae are completely ossified, and the 

 anterior ones confluent into one continuous mass. 



In the majority -of Chondropterygians the extremity of the 

 vertebral column shows a decidedly heterocercal condition 

 (Fig. 31), and only a few, like Squatina and some Eays, possess 

 a diphycercal tail. 



The advance in the development of the skeleton of the 

 Chondropterygians beyond the primitive condition of the pre- 

 vious sub-classes, manifests itself further by the presence of 

 neural and heemal elements, which extend to the foremost 

 part of the axial column, but of which the haemal form a 

 closed arch in the caudal region only, whilst on the trunk 

 they appear merely as a lateral longitudinal ridge. 



The neural and haemal apophyses are either merely 

 attached to the axis, as in Chondropterygians with per- 

 sistent notochord, the Eays and some Sharks ; or their basal 



/^f'^ 



/ / u ' 



Fig. 32. Fig. 33. Fig. 34. 



Fig. 32. — Lateral view. Fig. 33. — Longitudinal section. Fig. 34. — Transverse 

 section of Caudal vertebra of Basking Shark (Selache maxima). (After Hasse.) 

 a, Centrum ; b, Neurapopliysis ; c, Intercrural cartilage ; d, Haemapophysis ; e, 

 Spinal canal ; /, Intervertebral cavity ; g, Central canal for persistent portion of 

 notochord ; h, Haemal canals for blood-vessels. 



portions penetrate like wedges into the substance of the 

 centrum, so that, in a transverse section, in consequence of 



