DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
227 
and the great partition is complete from front to back (Plate 33, fig. 5, al'.n., s.n., p.e.); 
that tract, now partly bony (p.e.), is almost tliree-fourths the length of the whole 
basicranial axis, for a time, in this young suckling. The huge rounded intertrabecular 
base of the septum is, in the figure, largely hidden by the vomers, whose swelling 
form, however, tell of the bulk of the mass of cartilage covered by them. The septum 
nasi (s.n.) is low, and the perpendicular ethmoid ( p.e.) rises to no great height 
between the large cribriform plates ( cr.p .). There is no cartilaginous crista galli; 
from the free margin of the partition, a little forwards and then backwards, two-thirds 
of the way to the presphenoid (p.s.), the perpendicular ethmoid is bony, already. 
Bone is now seen in the base between the two orbitosphenoids; this is the pre- 
sphenoidal region ( p.s.), doubtfully or only partially independent as a bony centre. 
The orbitosphenoids (o.s.), and- alisphenoids (al.s.) are scarcely seen at all in this 
internal sectional view, they lie down so low, infero-laterally. But their upper margin 
is shown, and in the other lateral view (outside, fig. 3) their structure and relations 
can be seen. 
The optic nerve (fig. 3, II.) emerges above and inside the wide sphenoidal fissure 
(V 1 , 2 ) ; its passage through the middle of the proximal part of the orbitosphenoid 
was shown in the last stage (Plate 32, fig. 5, o.s., II.). The orbitosphenoid is also 
seen—in the distance—in the lower view (Plate 33, fig. 1, o.s.) bound upon by the 
orbital plate of the frontal (/.). 
The posterior sphenoid forms a very large part of the endocranium, and attains 
here its fullest development as a special (Insectivorous) type. The overlapping of 
the orbitosphenoid by the alisphenoid is seen well in the side view (fig. 3, o.s., al.s.), 
and in that aspect the foramen ovale (V 3 .) and the additional hole or hind opening of 
the alisphenoidal canal (al.s.c.) in front of it also ; the front overlapping lamina mounts 
up, and is seen under the junction of the frontal parietal and squamosal; the external 
pterygoid process is aborted. Below (fig. l), the posterior sphenoid is seen to extend 
from the place where the maxillaries overlap the palatines in front, to the foramen 
lacerum posterius (IX., X.), behind ; thus this great region ends, behind, opposite the 
middle of the basioccipital (h.o.). This extreme front and hind extension is peculiar 
to the typical Inseetivora, and is due to the forward growth of the alisphenoid at a 
good distance outside the orbitosphenoid, so making the sphenoidal fissure a side 
passage to the skull; whilst, behind, the tympanic wings of the basisphenoid grow 
beyond their root, and thus extend that bone, backwards. 
There is still some cartilage at the mid-line in the fore part of the presphenoidal 
region (fig. 5, p.s.), but where the two orbitosphenoids have met further back the 
bony base is complete, and that tract has already coalesced with the fore part of the 
independent basisphenoid (b.s.) (see also Plate 32, figs. 4, 5, p.s., b.s.), the basi- 
sphenoidal region is full twice the extent of the presphenoidal. The remarkable 
hollow under the fore part of the basisphenoid of the Hedgehog (Plates 17, 20, and 21) 
is also seen here, and the pituitary floor (sella turcica) is perforated; this hole, 
2 g 2 
