CRANIUM. 127 



The occipital region is more sharply marked off from the vertebral 

 colvmm in the Raji than in the Squah. In correspondence with this the 

 occipito-spinal nerves (p. 138) are reduced or absent in Raji. In a few 

 cases the occipital cartilage extends back over the anterior vertebrae and 

 envelopes them {Carcharias), while in Mtistelus one vertebra is fused with 

 the skull. O 



The greater part of the roof of the skull is formed of cartilage, 

 but there is a well-marked median fontanelle in the roof of the 

 ethmoidal region. The space in the auditory cartilage in which 

 the otocyst lies communicates with a canal which contains the 

 aqueductus vestibuli and opens on the dorsal surface, either into a 

 depression in the cartilage which receives the opening of the 



Fig. 72. — Median section of the cranium of Heianchus, inner view (after Gegenbaur). Fora- 

 men for 1 vagus, 2 glossopfiaryngeal, 3 auditory, 4 facial, 5 trigeminal nerve ; 6 pos- 

 terior clinoid ridge ; foramen for 7 oculomotor, 8 trochlear, 9 optic nerve ; 10 fontanelle ; 

 11 rostrum ; 12 lateral process of ethmoid region ; 13 foramen for carotid ; 14 transverse 

 canal in skull base ; 15 notochord ; 16 foramina for spino-occipital nerves ; 17 neural 

 arch of the first vertebra with nerve foramina. 



corresponding canal of the other side (Squali) or on the surface 

 separately from the latter (Raji). 



The ethmoidal region is frequently prolonged in front of the 

 nasal capsules. In the Raji and a few Squali this continuation 

 is a well-marked process, the rostrum (very largely developed in 

 Pristis) ; in Squali it is, if present, usually confined to a slender 

 process, the prenasal process, with which two processes of the 

 nasal capsules may be connected. 



The labial cartilages are small cartilaginous rods in folds of 

 skin at the sides of the mouth, near the angle ; there are usually 

 two pairs above and one pair below. They are less developed 

 and less constant in the Raji. 



Visceral Afches (p. 61). — The mandibular arch is always 



