DEVELOPMENT OE THE SKULL IN THE MAMMALIA. 
185 
ossify the chondrocranial wall. Behind and below that bone, tlie large labyrinth, in 
this view, is unossified, and in the cartilage the semicircular canals can be traced 
( a.s.c ., h.s.c., p.s.c .). The epihyal ( e.hy .) is seen descending, with the facial nerve 
(VII.) emerging behind it; and from the posterior fissure, and from the condyloid 
foramen the 9th, 10th, and 12th nerves are escaping (IX., X., XII.). Parallel 
with the posterior canal ( p.s.c .), and a little behind it, the junction of the opisthotic 
with the occipital cartilage can be traced, and a little behind that the growing exocci- 
pital bone (e.o.); under it is the condyle (oc.c.). The gentle, general curve is formed 
by the parietal, interparietal, and supraoccipital bones (p., i.p.,s.o.) ; between the 
latter and its side bone, the exoccipital (e.o.), there is still much cartilage. 
In the lower view of a skull ah this stage, with most of the investing bones removed 
(Plate 27, fig. 2), we see how much advance has been made. The long snout ( ctl.n.), 
with its infero-lateral nostrils {e.n.), is followed by the labyrinth, proper, with the 
peculiar supporting and conjugating investing bones that lie beneath it or between its 
moieties. The premaxillaries ( px.) have very long laminar palatine processes {p.px.), 
(involving the antero-lateral vomers); these support Jacobson’s organ and cartilages 
( j.o ., rc.c.), and reach as far back as those tongue-shaped cartilaginous tracts. The 
vomer {v.) is set between and above the palatine processes of the premaxillaries, and 
its pointed fore end is hidden by them ; its keel begins in their angle, and is very 
short. The body of the bone first narrows and then suddenly widens between the 
converging floor-plates of the nasal labyrinth ( n.f. ). In the chink right and left of the 
recurrent cartilage (rc.c) the inferior turbinals ( i.tb.) can be seen ; then the gap widens, 
and the upper and middle turbinal folds, now ossifying, are exposed ( u.tb ., m.tb.). But 
where, as in Passerine Birds, the broad hinder part of the vomer connects the right 
and left ethmoidal masses, there at each edge there is a small, additional postero-lateral 
vomer ( v".), and outside it an osseous patch in the cartilaginous floor itself {n.f.). 
The lateral cartilaginous tracts—floor and side-wall—first become constricted, and 
then expand; becoming, indeed, the upper broad unossified part of the orbitosphenoid 
(o.s.). But the hind part of each half of the nasal labyrinth is seen to end, in a 
bulbous form, right and left of the forks of the vomer, above which the end of the 
perpendicular ethmoid {p.e.) passes into the short presphenoidal region ( p.s .), still 
unossified. The orbitosphenoidal bony centres (o.s.) can only be partially seen from 
this aspect (see fig. 1), being hidden in front by the end of the nasal floor {n.f.), and 
behind by the alisphenoids {al.s.). 
The posterior sphenoidal region is now well ossified ; it is exceedingly broad and 
thick. 
Referring to the early chondrocranium (Plate 25, figs. 2, 3, b.s., al.s.), we see that 
the basisphenoidal region, just where the parachordals pass into the trabecula; under 
the pituitary body, is greatly dilated, right and left. If two imaginary lines be 
drawn obliquely backwards and a little outwards from the sides of the presphenoid to 
the side of the narrow waist in the base, between the cochleae, then we shall get the 
MDCCCLXXXV. 2 B 
