ANATOMY OF HETERODONTUS: ENDOSKELETON 449 
I. AXIAL SKELETON 
1. The skull 
The skull, like that of elasmobranchs in general, is composed 
of: (a) A cranium or brain case to which the sense capsules 
are fused in the adult; and (b) a series of cartilaginous visceral 
arches which support the buccal and pharyngeal regions. 
a. The cranium or brain case in dorsal view (fig. 1) is roughly 
quadrilateral in shape, slightly bifurcated at the anterior and 
posterior margins, and constricted along the sides at the first 
and second thirds—the first of these indentations being much the 
more pronounced. ‘The cranium is a closed box except at the 
antero-dorsal end, where there is a large opening, the anterior 
fontanelle (F.) and at the posterior end, where is located the 
foramen magnum (f.m.) through which the spinal cord joins 
the brain. 
In the mid-dorsal line joining these two openings several struc- 
tures appear. These are, passing forward, a ridge, the occipital 
crest (o.cr.) which runs to the parietal fossa (p.f.); from the bot- 
tom of the latter the endolymphatic ducts lead to the ears. In 
front of this.pit is a slight elevation which sinks immediately 
into a long groove—the parieto-frontal groove (p-f.g.) which, in 
turn, broadens out into the anterior fontanelle. 
On each side of and running parallel to the parieto-frontal 
groove there is a row of foramina extending posteriorly to the 
level of the parietal fossa. Anteriorly the first two of these on 
each side are the ophthalmics, through the first of which passes 
the ophthalmicus profundus nerve (f.0.p.’), through the second, 
the ophthalmic division of the seventh nerve (f.0.VII"). Through 
the numerous and smaller perforations which follow pass branches 
of nerves, and through the succeeding large foramina, blood 
vessels. These openings terminate posteriorly at an elevation 
produced by the anterior oblique semicircular canal (a.o.s.) 
which with a similar elevation from the opposite side roughly 
forms a broad V enclosing at its apex the parietal fossa. Below 
this there is a large inverted lower V, the arms of which enclose 
the foramen magnum (f.m.), and the apex of which abuts 
