49 
Its outline, as seen from the front, is flat below with a slight, abrupt rise 
at midwidth, rounded at either side, and broadly lowered medially 
above, so that the height is least at the middle, indicating the position of 
the olfactory lobes, one at either side. 
Piercing the cranium from side to side where it is narrowest in the 
lower part of the orbitosphenoids is a passage of considerable size vertically 
in line with the anterior end of the basisphenoid. This passage occupied 
by the optic chiasma also communicates widely upward with the brain- 
cavity through the floor below the space for the cerebrum; it is in advance 
of, and at a slightly higher level than the pituitary fossa with which it is 
confluent behind. The external openings of the passage are oblique to 
the longitudinal axis of the skull, and set at an angle to each other, approach- 
ing each other in front, and facing decidedly downward and forward. 
They are oval in outline and nearly twice as long as high. The median 
opening above is 30 mm. wide transversely. It was by means of the 
lateral openings that the optic nerves (II) found exit. 
Behind and at a slightly lower level than the transverse perforation 
just described, and separated from it by a bar of bone about 14 mm. 
broad, is a large, almost circular foramen apparently for the common exit 
of the oculomotor (III) and trochlear (IV) nerves. It occurs in the back 
part of the decided concavity marking the lateral compression of the 
lower, front portion of the brain-case. It opens through the upper part 
of the side walls of the pituitary cavity (infundibulum) its upper curve 
being about on a level with the posterior rim of the pituitary fossa. Run- 
ning forward from the lower margin of the foramen is the suture defining 
the upper limit of the front part of the basisphenoid. Whether this 
opening is wholly within the alisphenoid, or, as its position suggests, bound- 
ed behind by the alisphenoid and in front by the orbitosphenoid has not 
been determined, complete coalescence between these two elements having 
apparently taken place. 
Behind the foramen for the third and fourth cranial nerves, at a some- 
what higher level and separated from it by a space of about 27 mm., is an 
opening of large size, the foramen ovale or trigeminal nerve (V) exit. 
This foramen occupies its characteristic position in the reptilian skull 
in front of the probtic. It lies for the most part within the alisphenoid 
with the probtic bounding it posteriorly, and is a short distance above the 
hinder end of the base of the flange directed outward from the upper 
portion of the basisphenoid. Its outer opening is larger than the inner 
one but its greatest diameter is attained within the thickness of the bone 
where the size of its passage through is considerably increased by excavation 
forming a fossa. 
This foramen is best preserved on the right side in the paratype of 
Edmontosaurus where externally it is longer than high, subtriangular in 
outline and highest in front, measuring 24 mm. in length and 18 mm. 
in height. On the left side of the specimen the opening has been reduced 
in height by crushing. Leading forward from it anteriorly is an open 
channel or groove which marks the position of the ophthalmic branch of 
the trigeminal nerve (V) (see also p. 52). The suture between the alisphenoid 
and the probtic reaches the opening posteriorly from above. 
In the floor of the brain-cavity, midway between and at a slightly 
lower level than the trigeminal nerve (V) exits, are two small openings. 
