248 MR. W. K. PARKER ON THE STRUCTURE AND 
through a large round notch in the middle of the hind edge of the bone, and the bone 
grows under the nerve for some distance in its passage to the lower jaw. The alee are 
set on to the base obliquely, and the suture is on a level with the flatter upper face of 
the basisphenoid (fig. 4). 
If there is no tympanic ala to the basisphenoid, that part being a separate “ os bullee ” 
(fig. 2, 0.0.), there is nevertheless a large tympanic ala (¢.al.s.) growing from the hinder 
and under part of the alisphenoid ; in form this hollow growth is like an ordinary 
tympanic annulus, being crescentic, and having a wide convex face and a ragged 
opening looking outwards. All this is truly Marsupial, and if these alee coalesce with 
the ossa bullze in the adult the conformity is perfect. 
The auditory capsules are extremely large, and the petrous region is well ossified ; 
but the mastoid region is almost wholly cartilaginous. The fore margin is convex 
and bulbous, where the capsule, by its cochlear region (ch/.) fits into the large inter- 
space between the alisphenoid and exoccipital (e.0.); but higher up the fore margin 
is notched and sharp in front of the ampulla of the anterior canal (a.s.c.), the upper 
margin is free, but the hinder has coalesced with the occipital arch, yet not so as to 
obliterate the boundary line.* 
In the upper view (fig. 4) the coiled cochlear region (ch/.), the meatus internus 
(VIL, VIII), and the base of the recess for the flocculus are seen, but the main 
part of that hollow is hidden by the subvertical part of the capsule, unossified, and 
showing the form of the arch of the great anterior canal (a.s.c.). In the side view 
(fig. 3) the large convex mastoid region shows the elegant sweep of the horizontal and 
posterior canals (/.s.c., p.s.c.), the latter giving a smooth, rounded form to the mastoid 
ce 
margin of the capsule, with no “ mastoid process.” Beneath (fig. 5), the prootie and 
opisthotic bones are wholly amalgamated, the fore edge of the capsule and the cochlea 
being one common tract of bone—probably never quite separate centres, but developed, 
as in many Mammals, in a generalised way, as also is frequently the case in the 
Anurous Amphibia. The 7th nerve (VII.) can be seen running in its canal, reappear- 
ing inside the tegmen tympani, and burrowing again, to escape through the stylo- 
mastoid foramen, close behind the epihyal (ey.). In this view also (fig. 5) the 
fenestrae (/s.0., fr.) of the auditory labyrinth are well seen, divided, as in Marsupials, 
by a wide opisthotic bony tract. Then comes the wide interspace, or foramen lacerum 
posterius (IX., X.), and the actually perforated exoccipital (e.0.) (XII.) for with the 
exception of the olfactory sieve, the hypoglossal nerve is the only one which passes 
through a proper endocranial foramen ; the 7th and &th perforate a sense-capsule, and 
not a part of the true cranium. 
The occipital arch follows the auditory capsule, finishing the skull in a smooth and 
* This embryo was probably ripe, or nearly so, yet the development of bone in the skull is very 
remarkable, and quite like the early ossification of the Marsupial skull; the sharp free edges of the 
infero-lateral bones have been formed by arrest of the ossifying process, combined with absorption of a 
large tract of chondrocranium. 
