126 



SUB-CLASS ELASMOBRAXCHII. 



In the middle part of the trunk region in the Notidanidae, each centrum 

 carries two sets of neural arches and intercalated pieces, and corresponds 

 to two pairs of spinal nerves, and therefore to two segments. 



The haemal arches, which are sometimes supplemented by 



intercalated pieces, are in the trunk directed outM'ards, and 



carry very short ribs ; in the tail they are directed ventralwards, 



and meet below the caudal artery and vein. 



In the Raji the anterior vertebrae are not separate, but form a continuous 

 cartilaginous structvu-e into which the notochord extends only for a short 

 distance and which articulates with the occipital region of the skull. In 

 Notidanidae the anterior part of vertebral column has fused to the skull. 



The cranium, of which a general description is given on p. 60, 

 consists of continuous cartilage, which may be partially calcified. 



Fig. 70. — Three trunk vertebrae of 

 Alopecias vulpes (from Gegen- 

 baur, after Hasse). iv inter- 

 vertebral ligament ; n neural 

 arch ; in intercaleated piece of 

 neural arcli, m of haemal arch 



n- 



m 



Fig. 71. — Lateral view of a portion of the 

 caudal vertebral column of Rhinoptera 

 (Zygobates) (after Hasse), showing large 

 number of e.xtra dorsal pieces above the 

 intercalated pieces in and the neural 

 arches n. The neural arches are con- 

 tinued into the haemal k round the centra. 



The notochord may be entirely atrophied in the parachordal 

 region (Raji, etc.) or it may persist to a greater or less extent 

 {Heptanchus, Centropliorus, Acanthias, Squatina, Prionodon). 

 In Raji there is a distinct articulation between the occipital 

 region and the anterior vertebral plate of the vertebral column 

 by means of two occipital condyles and an ' odontoid process ' of 

 the vertebral plate. In the Squali there are also usually two 

 occipital condyles, but no regular joint between the first vertebra 

 and the skull. In the Notidanidae, in which the anterior end of 

 the notochordal sheath is not segmented, the latter structure 

 runs continuously into the cartilaginous region of the skull. 



