VERTEBRAE AND RIBS 



31 



tion occurs under the bases of the arches (and in some other places) 

 and then sections show radiations of calcified cartilage (astero- 

 spondylous vertebrsB, fig. 32, D). Cetorhinus has both radial and 

 concentric calcifications. 



Both neural elements, cranial and caudal, extend partly or 

 entirely around the spinal cord, but develop to varying extents in 

 different genera. The caudineurals form the neurapophyses, the 

 ventral nerve roots passing through them or between them and the 

 following cranineurals. Usually the cranineurals (intercalaria) do 

 not extend dorsally as far as do the caudineurals, but may meet 

 above the spinal cord, closing in the spinal canal (fig. 33), but as a 

 rule their upper ends remain at a lower level, while a roof-plate 

 cci crli) 



Fig. 33. Fig. 34. 



Fig. 2S. — Sagittal section of Acanlhias vertebra, cut surfaces lined obliquely. 

 c, calcifications of centra; cd. caudineurals; cdh. caudihffimals; cr (/), cranineurals (inter- 

 calaria); crh, cranihaemals; n, notochord; sc, spinal canal. 



Fig. 34. — Vertebrse of Heterodontus at junction of trunk and tail (Daniel, '15). 



covers the spinal cord in the intervals between the neurapophyses. 

 The dorsal nerve roots are usually enclosed in the intercalaria. 



The haemal arches are usually complete in the anterior caudal 

 region, but intercalaria are rare, and the haemapophyses are but slight 

 projections from the centra, either side of the dorsal aorta, and in 

 front they may rise to the level of the neurapophyses. Diplo- 

 spondyly and even polyspondyly often occur in the tail, there being 

 two or more sets of vertebral elements to a pair of muscles and spinal 

 nerves. Some evidence suggests that this is due to the obliteration, 

 in phylogeny, of nerve and muscle segments, the vertebra; remaining. 



Fusion of vertebras is frequent at the anterior end of the column, 

 especially in skates where fused segments afford support to the pec- 

 toral girdle. In sharks the column is firmly attached to the 

 skull, but in skates and Holocephals an articulation occurs 

 between them. 



