210 UPPER CRETACEOUS-TELEOSTS 
artery is visible. Posteriorly the parasphenoid articulates with the ventro-lateral 
edges of the basioccipital, ending posteriorly in two short processes below the hind 
end of the basioccipital. 
The basioccipital, posteriorly, forms the lower part of the occipital condyle and 
laterally it contacts the exoccipital above and the prootic in front. At the rear end 
of the lateral face of the basioccipital a pitted depression marks the point of insertion 
of supracleithral ligaments. Internally the basioccipital forms the posterior walls 
of the otolith chambers, and also separates them medially. The ventral surface of 
the basioccipital is deeply concave where it forms the roof to the myodome. The 
myodome is a deep cone-shaped recess ending blindly at the posterior end of the 
basioccipital. Ventrally, below the trigemino-facialis chamber, the anterior end of 
the otolith chamber is formed of prootic and is separated from the myodome by a 
membranous sheet. The otolith chamber is continued anteriorly as a recess on the 
internal face of the prootic. 
The exoccipitals contact each other in the mid-line of the posterior face of the 
skull, enclosing the foramen magnum completely. Ventrally each exoccipital con- 
tributes to the formation of the occipital condyle. Ventro-laterally the exoccipital 
is perforated by a large foramen through which the vagus nerve passes. Antero- 
ventrally the exoccipital contributes to the formation of the wall of the otolith 
chamber. The exoccipital, near its anterior edge, has a small foramen which 
transmits the glossopharyngeal nerve. The intercalar is entirely superficial in 
position, taking no part in the formation of the cranial wall itself. It is wrapped 
around the postero-ventral corner of the neurocranium beneath the opening of the 
post-temporal fossa. The intercalar is produced into a thickened knob of bone for 
the articulation of the ventral limb of the post-temporal. 
Ventrally within the orbit the basisphenoid is Y-shaped with a long, incompletely 
ossified pedicel, and shorter well ossified dorsal arms. These arms attach to the 
medial edges of the prootic and the anterior edge of the prootic bridge. The orbital 
face of the prootic contains the large anterior opening of the pars jugularis. Medial 
to this opening a further small foramen is visible which transmits the oculomotor 
nerve. Dorso-laterally the prootic meets the sphenotic and dorso-medially the 
pleurosphenoid. The pleurosphenoids are separated in the mid-line by a small 
optic fenestra, bordered ventrally only by the basisphenoid. A slight groove 
extends from the anterior opening of the pars jugularis to pass dorsally across the 
face of the pleurosphenoid. This groove indicates the course of the superficial 
ophthalmic nerves towards the ventral surface of the frontal. 
The orbitosphenoid is large, flattened and in the anterior orbital region. Ventrally 
it almost meets the parasphenoid and anteriorly the lateral ethmoids. Posteriorly 
the orbitosphenoid contacts the pleurosphenoids. 
Circumorbital series. There are eight circumorbital bones, of which seven are 
associated with the infraorbital sensory canal. The eighth is the supraorbital 
which is not canal bearing. The antorbital is a small fragile slip of bone contacting 
the supraorbital postero-dorsally, and the lachrymal antero-ventrally. The lach- 
rymal is the largest bone in the series and overlaps the upper regions of the maxilla. 
