DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
187 
edge of this opisthotic centre the bony matter has run forwards, and a little outwards, 
to the front of the cartilage, inside the antero-lateral notch. 
An upper view of this dissected skull—peeled of most of its investing bones (Plate 
27 , fig. l) shows the continuity of the elongated nasal labyrinth with the large orbito- 
sphenoidal wing ( o.s.). The long unjointed double nasal tube is but little enlarged, 
although sinuous, up to the proper olfactory region ( al.e .); there, in the swollen part, 
the marks of the turbinal folds are seen outside. The fore brain and large olfactory 
lobes overlie the hinder part of the labyrinth, and then the roof ends abruptly, a long 
way in front of the end of the floor; there we see the large cribriform plate ( cr/p.) 
between the roof and floor, in two hollows, greatly perforated, and separated by the 
perpendicular ethmoid (p.e.), which gives off a short cartilaginous crista galli ( cr.g .), 
above. The partial ossification of the turbinals, within, is shown in the lower view 
(fig. 2) ; in this figure the cribriform plate and mesethmoid ( cr.p ., p.e.) have a small 
centre, in the middle. Behind, the ethmoidal wall thickens, and right and left of 
it the end of the capsules is swollen in front of the orbitosphenoidal bony centres. 
These two bony tracts (o.s.) are subarcuate, narrow at their origin, and broader above; 
where the cartilage enlarges suddenly, there they end near their hind margin ; half-way 
outwards they are perforated by the small optic nerve (II.). The short and narrowish 
presphenoidal tract ( p.s.) is not ossified by them at present. Behind these parts the 
basisphenoid ( b.s .) is altogether abnormal, as compared with that of a Marsupial; it 
reaches nearly as far outwards as the orbitosphenoids, and has the small ear-shaped, 
biperforate alisphenoids ( cil.s.) placed on its edges. 
From side to side the basisphenoid is marked off into three nearly equal regions ; 
the two outer of these have a rounded hind margin; behind the fossae marking them 
off from the middle the cochleae (chi.) wedge in. Between the cochleae and the ali¬ 
sphenoids there is a membranous space; and between the basisphenoid and the basi- 
occipital (b.o.) there is a considerable tract of cartilage; here, at its narrowest, part, 
the basis cranii is very broad. Right and left the great misplaced prootic (pr.o.) is as 
well seen as in the lower view; it has a vertical position, for the cartilage further back 
turns inwards at its inner edge ; the opisthotic bony centre has crept outwards from 
the well ossified cochlea (chi.) ; its elegant coils are seen in this view, and behind them 
the archway and recesses for the 7th and 8th nerves (meatus interims, VII., VIII.). 
Backwards, and a little outwards from that passage, the huge anterior canal (a.s.c.) is 
seen, and has its walls bony, but the recess for the flocculus ( fi.r.) and the cartilage 
round and behind the great canal, are still unossified. In this view the basioccipital 
(b.o.) appears still broader and more reptilian than on the lower aspect; its fore part 
is still notched, where the bony cephalostyle was formed by the ossification of the 
cranial notochord. Right and left of the notched hind margin of the bone we see a 
large tract of cartilage separating the basal from the lateral bones—the exoccipitals 
(e.o.) ; the supraoccipital (s.o.), and the surrounding cartilage, has been cut aw T ay to 
expose the cranial floor. 
2 B 2 
