SKULL 



87 



from the Old Red Sandstone of Caithness possibly shows affinities 

 with this group. 



In Elasmobranchs the skull presents the simplest conditions 

 and most easily comprehensible relations, so that it may be taken 

 as the starting-point for the study of the skull of all other Verte- 

 brates. It consists of a simple cartilaginous and fibrous capsule 

 more or less movably articulated with the vertebral column, the 

 chondrocranium here reaching its highest development (Figs. 65 

 and 66), while from the Elasmobranchs onwards it undergoes, on 

 the whole, a gradual reduction owing to the increasing import- 

 ance of the osteocranium. The skull may become more 

 or less calcified, but true bones are never formed. The fibrous 



V.JO 



r 



FIG. 66. SKULL OF Chinuxra monstrosa, LATERAL VIEW. (From 

 Parker and Haswell's Zoology, after Hubrecht. ) 



a.s.c, position of anterior semicircular canal ; rh.y, ceratohyal ; ep.hy, epi- 

 hyal ; fr.cl, frontal clasper ; h.n.c, position of horizontal semicircular canal ; 

 i.o.s, interorbital septum; tb. 1, Ib. 2, Ib. 3, labial' cartilages ; Mck.C, 

 mandible ; No. 2, optic foramen ; Nv. 10, vagus foramen ; olf.cp, olfactory 

 capsule; op.r, opercular rays; pal.qn, palatoquadrate ; ph.hy, pliaryngo- 

 hyal ; p.x.c, position of posterior semicircular canal; qu, quadrate region; 

 r, rostrum. 



portions (fontanelles) are most marked in the prefrontal region, 

 except in the tropibasic skull of the Holocephali, in which there is 

 no prefrontal fontanelle and the interorbital region consists of a 

 thin membranous septum between the large eyes (Fig. 66). 



As in all Vertebrates above Cyclostomes, an assimilation of 

 vertebral elements has taken in the occipital region, so that the 

 nerves belonging to the vagus-group perforate the skull ; the part 

 of the skull situated posteriorly to these foramina has therefore 

 been described as a " neocranium " (cf. p. 85). 



The nasal region is often elongated to form a cut-water or 



