180 ME. W. P. PYCRAFT ON THE MOKPHOLOGY AND 



The Cranial Cavity. — The metencephalic fossa in all, save Dinomis and JEpyornis, 

 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-otic 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 (vi.). 



In yEpyornis and Dinomis the floor of this fossa is almost flat. 



The cerebellar fossa is roofed by the parietal and supra-occipital bones. The pro- 

 otics 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 masteidy monograph, states 

 that in Dinornithidce 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 Dinornithidce. It is of considerable depth in the 

 other Palceognathce and in Tinamous. 



The mesencephalic fossa is a deep basin-like depression in all save Dinornithidce and 

 Apteryx. 



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-otic. 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-otic, 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 

 Dromceus 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 Dinornithidce 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 Casuarius, is tunnel-like, and bounds the fossa anteriorly, 

 separating it from the pituitary fossa. 



