174 ME. W. P. PYCRAFT ON THE MORPHOLOGY AND 



processes ; the latter is more often slit-like and lies in the middle of the basitemporal 

 platform. 



The parasphenoidal rostrum, which is of great length, is thin and rod-like in 

 Apterycc, Casuarius, Bhea, Dromceus, more or less inflated in the Dinornithidce, Struthio, 

 and Tinamous. 



The Lateral Surface of the Cranium. — The tympanic cavity is moderately deep in 

 all save Aitteryx, in which it is comparatively shallow. The mouth of the cavity is 

 somewhat lozenge-shaped in Dinornithidce, ^pyornis, and more or less circulur in the 

 other forms. 



It is bounded posteriorly, in every instance, by the base of the paroccipital processes, 

 save only in the Tinamous, where, on account of the greater relative smallness of the 

 process, the tympanum is stretched along its free outer border. It is bounded 

 externally by the free inferior border of the squamosal. In all save Rhea and the 

 Tinamous, this border represents the posterior free edge of the zygomatic process. 

 In Ehea this process has shifted somewhat further iorwards, and its place is taken by 

 a second but much shorter. This second process may be regarded as a flange-like 

 downgrowth of the squamosal. The posterior region of this cavity lodges the apertures 

 of the internal ear ; the anterior, in front of this, is continued forwards as a long, 

 spacious, tunnel-like pneumatic cavity to form the anterior tympanic recess. This 

 aperture is largest in Bhea, Dromceus, and Casuarius; it is much smaller in jTJpyornis. 



The pre-temporal or alisphenoidal wing of the parasphenoid bounds the aperture of 

 this recess in front, and the pro-otic forms its roof, the basi- and exoccipitals share in 

 the formation of its posterior and ventral walls. The basitemporal plate and the 

 highly pneumatic basisphenoid receive the flnal termination of this recess beneath 

 the pituitary fossa, as is shown when the skull is seen in section. The bones taking 

 part in the formation of this cavity can of course only be made out in the skulls 

 of very young nestlings. Each recess curves gently forward to meet its fellow of the 

 opposite side. 



In Apteryx this recess is exceedingly small, and appears to lie, for the most part, if 

 not entirely, within the basisphenoid. There is no intercommunication between the 

 two recesses. 



The roof of the tympanic cavity is formed for the most part by the large articular 

 surface of the quadrate. Behind this is a conspicuous cavity, the roof of which is 

 perforated by numerous pneumatic apertures leading into the diploe Ixtween the pro- 

 otic and squamosal. It represents the superior tympanic cavity. Mesiad of this is 

 the fenestral recess containing the fenestra ovalis and rotunda and the aperture of 

 the posterior tympanic recess. In the larger Dinornithidce there is a small superior 

 tympanic recess. 



The Squamosal Prominence. — This is bounded in front by the temporal fossa, behind by 

 the paroccijiital process. It passes dorsad into the parietal, and is continued downwards 



