356 PROFESSOR OWEN ON THE GENUS DINORNIS. 
the third division of the trigeminal nerve, to form the articulation for the double 
condyle of the tympanic. The cavity is oval, directed from behind, forward and out- 
ward ; the postero-internal division is the largest and shallowest; the antero-external 
one is the deepest, and is hemispheroid. ‘The articulation is close and deep, whereby, 
with a peculiar suspensory structure, the tympanic is retained on the right side of the 
present skull, where the surrounding parts of the cavity are entire. Between the 
articular cavities for the tympanic and the paroccipital are the large pneumatic vacuities ; 
and behind them is a smooth, transversely extended deep fossa. This is partially 
bounded externally by the normal mastoid process, but is continued outward, con- 
tracting into the groove between the mastoid (Pl. XL. fig. 1,8) and the mesomastoid 
(s’) process. This process has contracted a filamentary bony union with the expanded 
base of the alisphenoid, the filament passing behind the neck of the tympanic, and 
helping to suspend and maintain it im situ. But the great characteristic of this part 
of the skull of Aptornis is the large size and advanced position of the premastoid pro- 
cess (ib. s+). This diverges, like the two hinder ones, from the common diapophysial 
basis, developed in and from the primordial cranial cartilage, and retaining all the 
guiding homological characteristics of the Crocodile’s mastoid. The process (8*), re- 
peating the zygomatic development thereof, is three-sided; the fore and hind sides are 
broadest, and meet externally at a sharp ridge directed downward and forward, bisecting 
the temporal fossa, but leaving the hinder division widest. Near its lower end the pre- 
mastoid contracts a bony union with the postfrontal (12) as in Aptornis otidiformis, but 
with a shorter, broader, and more compressed free termination beneath that union. 
The inner, narrow side of the premastoid process presents a rough surface for appa- 
rently ligamentous union with a process from the outer side of the tympanic. 
The basisphenoid is pierced anteriorly by the outlets of the Eustachian canals, con- 
tinued into each other by converging channels, underlapped by the process (Pl. XL. 
fig. 3, m), which is compressed and slightly produced downward. Above the Eustachian 
channels the basisphenoid contracts, and is produced into the presphenoid, first sending 
off on each side a small, horizontally compressed ridge or plate representing a “‘ ptera- 
pophysis,” but without an articular surface. If it is joined at all to the pterygoid, it 
must be only by ligament. 
Each orbito-sphenoid shows three small foramina in a transverse line, representing the 
“« prelacerate fissure,” behind the large optic foramen. ‘The optic foramina are barely 
two lines apart anteriorly. Beneath them the connate orbito-presphenoid is impressed 
by a smooth, oblong cavity, and is continued forward, confluent with the prefrontal, as 
an entire but thin “‘interorbital septum,” expanding anteriorly to form the back part of 
the olfactory chamber. ‘The rhinal platform (Pl. XL. fig. 3) is subcompressed vertically, 
The olfactory chambers do not extend backward beyond the “ platform ;” they are 
subspherical, half an inch in diameter, divided by a thin but complete septum, and 
perforated each by a single olfactory foramen a little external to their hind walls. 
