186 MR. W. K. PARKER ON THE STRUCTURE AND 
true width of the basal beam. Such a width is kept for the basisphenoidal ossifica- 
tion in the Marsupials, whilst all the part outside that definite bar is ossified by the 
alisphenoid, which also takes in the whole region that is left bare in the Mole between 
its alisphenoid and cochlea. Thus, in them, that thickening of the cartilage which is 
the foundation of the tympanic ala is ossified by the alisphenoid, and the tympanic 
wing has a large ‘‘os bullz” attached to it, behind and towards the mid-line. Here, 
as in the Hedgehog, there is no os bulle, and the dilated pneumatic part of the 
». 
posterior sphenoid is ossified by the abnormally large median centre (b.s.).* 
Behind the presphenoidal cartilage (Plate 27, fig. 2, p.s.) the ossified basal beam is 
seen to be flanked, right and left, by an outgrowth, which grows into a lower plane, 
and stretches outwards as faras to the under and inner edge of the alisphenoid (a/.s.). 
These suboval masses look inwards in front, and are notched in that part ; their inner 
margin is swollen and rounded, their outer and harder edge is sinuous. A large air 
cell is forming on the outer part of the under surface, and the whole mass is spongy. 
This very ornithic condition of the dilated and pneumatic basisphenoid is only a 
modification of the parts quite similar to what I have just described in the Hedgehog, 
where, however, the bones are more solid, and are devoid of this peculiar spongy 
growth. Resting on this wide, pneumatic basisphenoid, we see the narrow oblongo- 
crescentic alisphenoids with their large foramina—foramen ovale and foramen 
rotundum (V*., V*.); the outer front corner of each bone lies under the corresponding 
orbitosphenoidal cartilage (0.s’.). The rest of the cranium proper, is seen wedged, in 
between, and expanding behind, the large auditory capsules. The synchondrosis is 
lessening fast, and the rest of the parachordal region is occupied by the very Reptilian 
basioccipital (b.0.). This relatively large plate is polygonal, has a notched fore edge, 
a concave hind margin, and roughly-sinuous sides ; it is concave, right and left of the 
mid-line. Behind it we see the huge, flattish condyles (oc.c.), which are reniform, 
having a large notch on their inner edge; outside these we see the creeping ossi- 
fication of the exoccipitals (¢.0.), and outside these a very low and narrow paroccipital 
tract (p.oc.). The perforating foramen condyloideum (XII.), and the hinder foramen 
lacerum (IX., X.) in front of, the occipital arch, are here seen. 
The deeply notched antero-lateral margin of the great ear-capsule is bounded exter- 
nally by the rambling bony growth of the prootic (pr.o’.). The inner edge of that 
rounded notch, and the rest. of the opisthotic region behind it, and behind the cochlea 
(chi.) is still unossified, and shows in this aspect the horizontal and posterior canals 
(h.s.c., p.s.c.); shining through its semitransparent substance. The cochlea and the 
contiguous part of the vestibule (ch/.) are well ossified; the fenestre are at right 
angles to the general basal plane, and are not well seen in this view. From the outer 
* Tn this remarkable pneumaticity of the basal and sub-basal parts of the skull, in relation to the 
tympanic air-cavity, the fundamental structure is quite similar in the Crocodile, Bird, Marsupial, and 
Tnsectivore: the after-modification gives the diagnostic, in each case. 
