180 MR. W. P. PYCRAFT ON THE MORPHOLOGY AND 
The Cranial Cavity—The metencephalic fossa in all, save Dinornis and Apyornis, 
takes the form of a moderately deep basin, the floor of which is tilted upwards to a 
very considerable extent. The anterior tilted portion forms the posterior wall of the 
pituitary fossa. Posteriorly it is continued backwards on to the occipital condyle. Its 
postero-lateral border, immediately below the pro-dtic is pierced by the vagus foramen, 
to the inner side of, and slightly posterior to, which lie one or two condyloid foramina. 
Mesiad of the internal auditory meatus, and anterior to the same, is the abducens 
foramen (V1.). 
In Zpyornis and Dinornis the floor of this fossa is almost flat. 
The cerebellar fossa is roofed by the parietal and supra-occipital bones. The pro- 
dtics bound it laterally, ventrally it passes into the metencephalic fossa. It is bounded 
behind by the free edge of the occipital foramen, and in front by the median portion of 
the tentorial ridge. The late Prof. Jeffery Parker, in his masterly monograph, states 
that in Dinornithide the supra-occipital region of this fossa is marked by “ transverse 
grooves corresponding with the gyri of the cerebellum.” Immediately above the 
internal auditory meatus lies the floccular fossa. The size of this, according to Parker, 
appears to vary individually in Dinornithide. It is of considerable depth in the 
other Paleognathe and in Tinamous. 
The mesencephalic fossa is a deep basin-like depression in all save Dinornithide and 
ApteryLZ. 
It appears to be more sharply defined in Casuarius than in any other member of this 
group. It is bounded above by a very prominent shelf of bone, forming the lateral portion 
of the tentorial ridge, behind by the pro-étic. It extends forwards considerably 
beyond the level of the pituitary fossa which bounds it in the middle line. ‘The 
trigeminal foramen (v.) leaves by a large aperture excavated out of the ventral border 
of this fossa, between it and the pro-dtic, and is of considerable size. Just within the 
mouth of this foramen, below the rim of its anterior border, lies the aperture of the 
orbito-nasal nerve (v.'), which has its exits in an almost obsolete lacerate fossa. In 
Dromeus the trigeminal and orbito-nasal foramina are distinct. The tentorial ridge, 
bounding the fossa superiorly, is almost as strongly developed as in Casuarius. 
In Struthio this fossa is relatively smaller and shallower, neither are its boundaries 
so sharply defined. The apertures of the trigeminal foramen and the orbito-nasal 
nerve lie close together. 
Rhea and Crypturi have the fossa somewhat more sharply defined than in Struthio, 
but in none of these does that portion of the tentorial ridge bounding the fossa 
superiorly form anything more than a low ridge. 
In Dinornithide this fossa is relatively ill-defined and shallow. The aperture for 
the trigeminal foramen steals away a large portion of its external wall. The orbito- 
nasal aperture opens as in Caswarius, is tunnei-like, and bounds the fossa anteriorly, 
separating it from the pituitary fossa. 
