174 MR. 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 
Apteryx, Casuarius, Rhea, Dromeus, more or less inflated in the Dinornithide, Struthio, 
and Tinamous. 
The Lateral Surface of the Cranium.—The tympanic cavity is moderately deep in 
all save Apterya, in which it is comparatively shallow. The mouth of the cavity is 
somewhat lozenge-shaped in Dinornithidw, dpyornis, and more or less circular 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 Rhea this process has shifted somewhat further forwards, 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 Rhea, Dromeus, and Caswarius; it is much smaller in Zpyornis. 
The pre-temporal or alisphenoidal wing of the parasphenoid bounds the aperture of 
this recess in front, and the pro-étic 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 final 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 diploé between the pro- 
Stic 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 Dinornithide there is a small superior 
tympanic recess. 
The Squamosal Prominence.—This is bounded in front by the temporal fossa, behind by 
the paroccipital process. It passes dorsad into the parietal, and is continued downwards 
