158 " MR. D. M. §. WATSON ON THE 
very large area of the outer surface. In advance of them a 
narrow rostrum, which is probably to some extent parasphenoid, 
projects forward and articulates with a special facet on the 
dorsal surface of the pterygoids so as to divide completely the 
interpterygoid vacuity. 
Although the lower surface of the basisphenoid is very long its 
upper surface is short, owing to the great anterior production of 
the basioccipital. The lateral border of the upper surface and a 
good deal of the side of this bone are covered by the prootic, In 
advance of this region the bone suddenly narrows, its sides being 
in contact with the sphenethmoid. The front of the bone ter- 
minates in a smoothly rounded edge, which is the back of the 
opening to the pituitary fossa. This runs backwards with sides 
widely open above the basipterygoid processes; its base is closed by 
the upper surface of the rostrum, and in front it passes into the 
cavity of the sphenethmoid. The internal carotids enter the 
pituitary fossa by a single foramen, which leads into a canal 
passing down in the body of the basisphenoid for about a centi- 
metre and then splitting into two, which pass out on the sides of 
the bone above the basipterygoid processes. In front of the 
pituitary fossa the rostrum 1s in contact with the under surface 
of the sphenethmoid. 
Otic bones.—The side walls and roof of the posterior part of the 
brain-case are formed by the fused otic bones and exoccipitals. 
The exoccipital part of the mass forms a massive pillar pierced 
near its base by a single small foramen for the XIIth nerve; its 
anterior border forms the back of the huge jugular foramen, 
which is about 1°5 em. in diameter; it must form part of the very 
powerful paroccipital process, and it is not improbable that the 
two exoccipitals met above the foramen magnum. 
The front border of the jugular foramen must be formed by 
the opisthotic, which is so fused with the prootic and supra- 
occipital that it is hopeless to separate them ; the three bones 
with the exoccipital form a very massive paroccipital process, 
whose outer end is firmly united with the tabular and squamosal 
by an irregular suture seen only in fractures. 
The foramen jugulare issues below and behind the process, and 
on its lower front surface, not very far out, is the fenestra ovale, 
a large hole of irregular shape. 
In section, on the left side, the horizontal semicircular canal 
and part of the vestibule are seen filled with matrix. 
The foramen through which the VIIIth nerve gains entrance to 
the ear lies far out on the front wall of the bony canal, which 
forms the foramen jugulare. The foramen is a small one, the 
vestibule being separated from the brain-cavity by bone and not 
merely a membrane, as in most early reptiles. 
The aqueductus fallopii for the VIIth nerve has an exit on the 
outer surface of the prootic in advance of and slightly above the 
fenestra ovale; its opening to the brain-cavity is not quite 
certain, 6 
