VERTEBRA AND RIBS 



29 



of the centra farther forwards and may be borne anteriorly even at 

 the level of the neural arch. 



The vertebrae are never ossified in Elasmobranchs, but may be 

 calcified (p. 6). In Chondrostei bone may be deposited on the 

 outside of the cartilage; and every stage may be traced from this to 

 conditions in which practically all of the cartilage of the vertebra is 

 replaced by bone. 



ELASMOBRANCHII have considerable variety in vertebrae. 

 Possibly the most primitive is found in the extinct Xenacanthidae 

 (fig. 29) where several elements (but no centra) appear to each 

 somite, these being apparently comparable to the parts in the better 

 known cartilaginous Ganoids and referable to the neurals and haemals 

 of the developmental stages of other Elasmobranchs (p. 19). 



Fig. 29. Fig. 30. 



Fig. 29. — Five caudal vertebrae of Xenacanthus (Fritsch). h, haemal arches; w, 

 neural arches; nc, position of notochord. 



Fig. 30. — Vertebrae of ChimcBra (Schauinsland). cdn, cm, caudi- and cranineurals; 

 dr, dorsal roots of spinal nerves; h, haemals; n, notochord with calcifications; vr, ventral 

 roots of spinal nerves. 



Of living Elasmobranchs the Holocephali, so far as vertebrae 

 are concerned, are the most primitive (fig. 30), while Notidanids, 

 Echinorhinus and LcBmargus are scarcely more advanced. Holo- 

 cephals develop the eight parts described above to a somite. In the 

 trunk only the caudineurals form neurapophyses, the cranials 

 (intercalaria) remaining small, their bases resting on the notochordal 

 sheath, the upper margin rising but slightly on the side of the spinal 

 cord. Each cranineural grows around a ventral root of a spinal 

 nerve, the dorsal root passing out between the successive neura- 

 pophyses (fig. 30). Fusion of haemals forms a plate on the ventral 

 side of the notochord. At first the neurapophyses do not meet 

 above the spinal cord, but do later, and the gaps between the succes- 



