136 



SELACHII 



from the outer layer (Raja). This outer layer is sometimes, at all 

 events, formed outside the elastica externa, by a spreading out of 



IV. 



ecu. 



FIG. 03. 



Inner view of the right half of the skull of Hexanchns. (After Gegenbaur.) a/-, foramen for 

 auditory, gp, for glossopharyngeal, o, for optic, ocn, for spino-occipital, om, for oculomotor, 

 tg, for trigeminal, tr, for troclilear, vg, for vagus, and vs, for occipit.o-spinal nerve ; a.p, 

 antorbital process ; t, carotid foramen ; en, interorbital canal ; MI, membrane over fontanelle ; 

 r, rostrum. 



the bases of the arches, which finally meet round the notochordal 

 centrum. A compound centrum is thus formed, partly 'perichordal' 



rU dr. 



FIG. 04. 



LaniiM cornubicu, Gm. A, portion of the vertebral column of the trunk, partly cut longi- 

 tudinally (right-side view). B, transverse section of the same through the middle of a centrum. 

 c, centrum ; cu, calcareous constricted ring ; d.r, foramen for dorsal nerve-root : ]ia, basiventra] : 

 in, interdorsal ; iv, interventral ; li, ligament ; na, basidorsal ; n.c, neural canal ; nt, notocliord ; 

 r.ca, radial calcifications ; v.r, foramen for ventral root ; w, intervertebral ligament. 



and partly 'chorda!' (p. 100). The ventral root of the spinal 

 nerves, as a rule, comes out either behind or through the neural 

 arch (basidorsal) ; the dorsal root either behind or through the 



