1910.] ANATOMY OF THE FRILLED SHARK. 543 



The cartilage of the floor of the cranium in the region of its 

 junction with the vertebral column is thick and somewhat heavily 

 calcified. It here shows some indication of its probable vertebral 

 nature, by the slight resemblance which the calcification presents 

 to the inverted V-formation found in the centra of the vertebral 

 column. 



The nerve foramina, as seen both from the inside and from the 

 outside of the skull, deserve some consideration. The foramen 

 for the first nerve (i.) is very large and lies just below the opening 

 of the anterior fontanelle. The optic nerve foramen (n.) is 

 moderately large and, seen from the inside, lies a short distance 

 posterior to the opening for the olfactory nerve. On the outside 

 it opens into a deep channel on the anterior dorsal side of the 

 smooth articular surface which receives the ethmo-palatine 

 process of the palatoquadrate. It is thus situated comparatively 

 far forward in the orbit. 



The foramen for the third nerve (in.) lies at the end of a for- 

 wardly curved groove on the inside of the skull. Externally it is 

 placed close to the floor of the cranium just behind the posterior 

 ridge of the above mentioned articular surface. 



The aperture for the fourth nerve (iv.) is small and lies well up 

 in the orbit, almost vertically above (in.). Behind foramen in. 

 is the cartilaginous optic stalk (p.s.), against the expanded end of 

 which the back of the eyeball rests. Immediately posterior to the 

 origin of this stalk is a fairly large foramen which transmits the 

 interorbital blood-sinus. On the inside of the skull, it is divided 

 by means of a thin, outwardly directed cartilaginous bridge. The 

 interorbital sinus passes posterior to this, whilst the cavity 

 anterior to it forms the pituitary fossa. The foramen of the 

 internal carotid artery (i.e./.) is a small aperture lying in the floor 

 of the skull immediately anterior to the pituitary fossa. 



Nerves five, six, and seven are transmitted by a very large 

 foramen (v. VI. & Vii.) which is about twice as long as broad, 

 and has a thin, backwardly directed ridge on its anterior edge. 

 Following this is a double foramen (vii. & viii.) on the inside of the 

 skull, which transmits the hyoidean branch of the seventh nerve 

 and also nerve eight. The hyoidean part is the anterior smaller 

 portion, which is continued directly outwards and opens externally 

 just beneath the postorbital process. The foramen for nerve nine 

 is small and is somewhat ventrally placed a short distance posterior 

 to foramen viii. It is continued obliquely under the auditory 

 capsule and opens externally at the back of the cranium in a very 

 deep depression, overhung by the occipital ridges. 



The tenth foramen is moderately large and on the' inside is 

 situated in the median line. It opens externally at the back of 

 the skull just above the aperture for the ninth nerve. There are 

 four small foramina, somewhat ventrally placed, the first one tying 

 immediately below foramen x. These are the foramina of the 

 spino-occipital nerves, so x -so*. 



