THYLOGENY OF THE I'AL.EOflN'ATHJE AND NEOGNATH.E. 197 



The pro-otic completely shuts out the squamosal from the interior of the skull in all 

 but Apterya'. In this type a large, roughly triangular plate appears immediately above 

 the pro-otic and wedged in between tlie parietal and alisphenoid. 



The floccular fossa is deep, and oblong in all but Apteryx, the long axis running 

 vertical to tiiat of the skull. In Aptery.v it is represented by a small but deep and 

 perfectly circular pit. 



Externally, the pro-otic appears only within the tympanic cavity and affords two more 

 or less distinctly separated articular surfaces for the quadrate : the one lying slightly 

 above and in front of the other. Distad of the superior glenoid surface lies a similar 

 articular surface furnished by the alisphenoid ; caudad of the inferior lies a second 

 facet furnished by the exnccipital. These appear to be most distinctly traceable in 

 CasuariKS. Externally, articular surface for the quadrate is completed by the squamosal 

 Thus thei-e are two pro-otic, one exoccipital, one alisphenoid, and one squamosal 

 articular facet for articulation of the quadrate. 



The opistliotic is seen as a small oblong nodule of bone wedged in between the pro- 

 otic and the exoccipital, and bounded above by the inferior end of the epiotic. It is 

 not visible externally. 



The foramen for the vagus lies at its ventral extremity and between it and the exoccipital. 



The hasis])]ienokl has in every case fused completely with the luiderlyingbasi-temporal 

 plate, and is therefore traceable only from the superior aspect of the skull. It is 

 moderately thick in section, the body of the bone being thickened by a mass of diploe. 



It is bounded behind by the basi-occipital, laterally by the pro-otic and alisphenoid. 

 Anteriorly, in Ehea, it is produced forward into a thin vertical plate of bone, to join the 

 cartilaginous presphenoid ; in the other forms this vertical plate is much shorter. It 

 serves to form the inner half of the circumference of the foramen for the orbito-nasal 

 nerve (v.), the outer portion being contributed by the alisphenoid. Out of its 

 anterior region is scooped the pituitary fossa. The abducent foramen pierces it on 

 either side near its postero-lateral angle. It forms the floor of the anterior region of 

 the metencephalic fossa. 



In Apteryx it is almost quadrangular. In Casuarim and Uroiiiaus, and in Ehea, it is 

 expanded laterally to form a pair of wings, and is narrower behind than in front. 



The alisphenoid is bounded by the parietal and squamosal behind, the orbital plate of 

 the frontal above, and the combined basisphenoid and alisphenoid as rings of the 

 parasphenoid. 



In iJronueus, Casuaritis, and B/iea it bears a share in the formation of the jiost-orbital 

 process. 



The alisphenoid lodges the greatei- part of the mesencephalic fossa, and contributes 

 a share towards the formation of the anterior wall of the cerebral fossa. 



The orhitosphenoid is represented only by cartilage and is much reduced in size. 



The presphenoid is represented by a thin vertical plate of cartilage resting upon th(> 

 VOL XV. — PART V. No. 12. — December, 1900. 2 i' 



