1808.] OSTEOLOGY Of THE I.MPEXXES. 967 



lateral region oE the dorsum sell^ may be said to define it 

 posteriorly. Its outer wall is pierced by the trigeminal foramen. 



The pituitary fossa is very deep ; its floor is pierced by two 

 foramina leading outwards above the Eustachian grooves, at the 

 point where the inner free border of the basispheuoid and para- 

 spbenoid plates meet one another to form the Eustachian tube. 

 The dorsum seilse is a flattened plate of bone sloping obliquely 

 forwards over the pituitary fossa, and terminating at the oculo- 

 motor foramen. The pre-pituitai-y ridge slopes gently forwards ; 

 anteriorly to this, in the middle line, is a small triangular optic 

 platform. The pre-optic ridge terminates on either side somewhat 

 above the level of the tentorial ridge. The anterior border of the 

 optic foramen is completed by the presphenoid, the posterior 

 border by the alisphenoid. 



The cerebral lies in front of the cerebellar fossa, the cerebellum 

 not being cohered by the cerebrum. The tentorial rises slightly 

 below the level of the pre-optic ridge, sweeps backwards to the 

 level of the junction of the epi- and proijtics, then almost vertically 

 upwards to the middle line, to terminate in the roof of the skuJl 

 above the region of the dorsum sellse. Erom this point forwards 

 it is continued as a sharply defined ridge losing itself in the 

 extreme anterior region of the fossa. 



iii. The Skull of the Nestling. 



The sutures of the skull, like those of btruthious birds, remain 

 open for a very considerable time, being quite distinct in advanced 

 nestlings. The Museum collection possesses two such skulls — one 

 of Oatarrhactes civnjsocome about quarter-grown, and one of a half- 

 grown PyrjosceUs papua (PL LXI. figs. 1-8) ; and from these the 

 following descriptions are taken. 



The occijpitoJ condyle is almost entirely formed by the basi- 

 occipital, only a small portion being contributed by the exoccipital; 

 a deep pit in its centre in the dried skuU represents the remains 

 of the notochord. 



The supra-occipital, seen from v^ithout, is a vertically elongated 

 bone of a rounded oval in outline and tttmid in shape. It con- 

 stitutes the characteristic " cerebellar prominence." It is bounded 

 above by the parietal, and laterally by the epiotic, from which, in 

 very young skulls, it is separated in part, superiorly, by a wide 

 chink, and in part, inferiorly, by a deep groove. Its inferior 

 border forms the upper boundary of the foramen magnum. There 

 are traces, in the earlier stages, of an originally paired condition, 

 in the shape of a mesial cleft running from the superior border 

 downwards for about g of its length ; it then bifurcates, the two 

 limbs terminating almost immediately after : later, as seen in a 

 young Pyfjoscelis, the median cleft closes up, leaving a horseshoe- 

 shaped fenestra representing the bifurcation, which, in its turn, 

 disappears leaving in the adult no trace. The deep inferior groove 

 separating the supra-occipital from the epiotic has in some casts 



