ESPECIALLY MYCTOPHOIDS 43 
fenestra between it and the basioccipital. This fenestra opens into the myodome 
posteriorly, the parasphenoid forming the entire floor of the posterior myodome. 
The basioccipital forms the whole of the occipital condyle. The condyle is circular 
and concave with a very small notochordal pit in its centre. Anteriorly the basi- 
occipital narrows and its undersurface passes antero-dorsally above the myodome 
forming part of its roof. As well as the posterior part of the roof of the myodome, 
the basioccipital also enters into the dorso-lateral parts of the wall of the myodome. 
On the lateral face of the basioccipital near to the condyle there is a group of pits and 
ridges. Internally the basioccipital forms the walls of the posterior parts of the 
otolith chambers, and as the ventral surface of the bone is inclined antero-dorsally 
above the myodome the otolith chambers become partially separated in the mid-line. 
The basioccipital contacts the rear edge of the prootic bridge anteriorly, and the 
otolith chambers extend forwards as narrow recesses within the prootics below the 
facial formen. 
The prootic is a large and extremely complex bone. Each prootic consists of two 
vertical sheets of bone joined anteriorly and ventrally. The inner sheet contacts the 
basioccipital behind, separating the myodome from the otolith chamber, whilst the 
outer sheet forms the wall of the otolith chamber, meeting the parasphenoid below 
and the exoccipital posteriorly. Dorsally the inner sheet curves medially meeting its 
fellow in the mid-line, forming the prootic bridge which separates the myodome 
from the cranial cavity. The lateral sheet curves dorso-laterally to contact the 
sphenotic and pterotic in the hyomandibular facet and the exoccipital posteriorly. 
These two divergent sheets of bone are connected anteriorly by a sheet of bone lying 
in the postero-lateral face of the orbit. This meets the sphenotic dorso-laterally and 
the pleurosphenoid dorso-medially, and its medial edge forms the margin of the optic 
fenestra. The trigemino-facialis chamber, within the prootic, is divided by a bony 
partition into a medial pars ganglionaris and a lateral pars jugularis. The pars 
jugularis is a long horizontal canal, widest anteriorly, lying within the prootic 
lateral to the prootic bridge and in the angle between the dorsal and ventral parts of 
the lateral face of the bone. Two foramina lead from the cranial cavity into the pars 
jugularis, the more posterior one being the facial foramen which transmitted the 
hyomandibular and palatine branches of the facial nerve. The anterior, trigeminal 
foramen transmitted the trigeminal and the buccal, otic and superficial ophthalmic 
branches of the facial nerve. The pars jugularis has four external openings. The 
posterior one is an oval foramen opening on the hind edge of the prootic, and this 
transmitted the jugular vein. Dorso-laterally, below the anterior region of the hyo- 
mandibular facet there is a second large dorso-ventrally elongated foramen which 
transmitted the hyomandibular branch of the facial nerve outwards from the pars 
jugularis. Below the jugular canal there is a third foramen which opens into a canal 
passing up into the floor of the pars jugularis, this canal transmitted the orbital 
artery into the jugular canal. The fourth foramen is the largest and is the anterior 
opening of the pars jugularis in the hind wall of the orbit. The trigeminal nerve, the 
buccal and otic branches of the facial nerve, the superficial ophthalmic components 
of both nerves, the jugular vein and the orbital artery all passed through this anterior 
