PHTLOGENY OF THE PAL.^OGNATH.E AND NEOGNATH^. 181 



In Aptery.v it appears to be reduced to the vanishing point. It is extremely small, 

 oblong in shape — its long axis liorizoutal, — and more or less sharply defined, when 

 seen at its best. It lies entirely behind the level of the pituitary fossa. It is 

 bounded above by a low tentorial ridge, in front by a ridge of bone dividing it from the 

 pituitary fossa, below by the rim of the metencephalic fossa, and behind by a huge 

 projecting shelf-like ledge from the tentorial ridge. The trigeminal foramen opens at 

 its postero-dorsal border, the orhito-nasal in its anterior border. 



In A. haasti the outer wall of this fossa is almost entirely absorbed by a large aperture 

 common to the trigeminal and orbito-nasal nerves. 



The ]}ituitari/ fossa in Casuarius is a deep pocket-shaped depressimi. Its mouth is 

 directed slightly forwards. The rim thereof forms the dorsum sellce behind, and the 

 pre-pituitary ridge in front. Within the mouth of this fossa [Linornis) or on its rim 

 (Drommis) is the oculo-motor (iii.) foramen. The pathetic (iv.) foramen lies above and 

 to the outer side of the oculo-motor, often it serves also for the passage of the internal 

 ophthalmic artery. Anterior to the prepituitary ridge in the middle line is a narrow 

 knife-like optic platform, on either side of which are the confluent o/jtic furariwui. 

 The optic platform is continued upwards vertically and again expands to tbrm the 

 pre-optic ridge, which in Dromwus, and slightly less obviously so in Casuarius, joins 

 the tentorial ridge. 



In Struthio tlie pituitary fossa is of great depth ; the pre-pituitary ridge and optic 

 platform are well marked. 



In Dinornithidce the fossa is comparatively shallow, but of considerable circumference. 

 The optic platform is very wide. 



In Ajjteri/x the pre-pituitary ridge is but feebly developed. The optic platform is 

 not distinguishable from the pre-pituitary ridge, which divides the optic foramina one 

 from another. The pre-optic ridge is well developed and overhangs the fossa, the 

 mouth of which, in consequence, has a semicircular outline. It should be mentioned 

 here that the bottom of the fossa is produced forwards into a yet deeper, almost 

 spherical, depression, overhung by the pre-pituitary ridge. 



In Tinamous this fossa is deep and tubular in form. 



The cerebral fossw are sharply defined in all the Pala-oguathce save Apteryx. In all, 

 with the exception just mentioned, the cerebral lies entirely in front of the cerebellai- 

 fossa. In Apteryx, and to a lesser extent in the Crypturi, the cerebellar contour is 

 scarcely distinguishable from the cerebral. The tentorial ridge, which serves to cut ofl' 

 the cerebral from the remaining fossae, arises from the level of the pre-optic ridge and 

 trends outwards and backwards to the level of the top of the pro-otic ; from this;^point 

 it turns sharply upwards and forwards to meet in the mid-dorsal line, over the region 

 corresponding with a vertical line passing upwards from the dorsum selliE. Ihat 

 portion of the ridge which forms the upper boundary of the mesencephalic fossa in 

 Casuarius and Drommis is of great depth, and projects into the cranial cavity like a 



VOL. XV.— FART V. No. lU. — December, 1900. 2 d 



