DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
53 
The alinasal fold ( al.n .) runs well round the end of the septum (s.n.), which 
shows the usual fenestra at this part ( i.n.f.); behind the snout the alee nasi give off 
from their base the usual spoon-like recurrent cartilage (rc.c.) for the protection of 
“Jacobson’s organ.” 
Two bones protect most of the top, but, below, the middle part, only, is supported by 
bone. The upper are the nasals ( n .), and frontals (f), the lower the vomer (v.); I did 
not find the “ postero-lateral vomers ” developed in this stage; when they do appear 
they do not remain distinct, for long : the antero-lateral vomers had evidently already 
coalesced with the premaxillaries (Plate 11, fig. 8). The floor of the deep olfactory 
recess, or cribriform plate (cr.p.) has been cut across; but it will be seen that this 
section leaves a large triangle of cartilage above and behind, making the recesses 
appear much deeper than they otherwise would do; this forms a large cartilaginous 
“ cristi galli ” or proximal solid part of the falx cerebri (see also in Tatusia hybrida, 
Plate 2, fig. 1, cr.g., where, however, it is not so large as in this type.)* 
The hemispheres are lodged in a strong box, already ; the fontanelle (fo.), and the 
triradiate suture between the frontal, parietal, and squamosal (f, p., sq .) are clearly 
seen, and the growing thickness of the bones, especially of the parietal, where it forms 
an attachment for the “ tentorium cerebelli ” in front of the anterior semicircular 
canal (a.s.c.). The falcate cartilage over the low bony orbito-sphenoid (o.s.', o.s.) still 
shows its distinctness and is still a considerable distance behind the cribriform plate, 
below. The bony orbitosphenoids (o.s.) are obliquely oblong and are turned forwards 
and outwards; the large optic foramen (II.), well surrounded by bone, lies low down ; 
the basal bar ( p.s .) is not ossified. A considerable fenestra (or fontanelle) still exists 
over the sphenoidal fissure (V 1, 3 .), not yet covered over by the frontal and squamosal. 
Under the latter the alisphenoid (al.s.) lies outwards, and leans forwards, letting the 
nerve of the lower jaw (V 3 .) pass over its edge at the hinder fourth ; a snag of bone 
protects the nerve behind, but above, it escapes through a notch. The basisphenoid 
(b.s.) is separately ossified, and has taken up much of the cartilage of its own 
territory; then comes a tract of cartilage three-fourths as long, and then a tract of 
bone, the basioccipital (b.o.). 
The latter bone comes far short of the foramen magnum (see Plate 11, fig. 8). The 
fore part of the spheno-occipital synchondrosis is thick, forming the low postclinoid 
wall. In the large “foramen lacerum” between it, the alisphenoid, the squamosal, 
and the round end of the large cochlea, the internal carotid artery enters. The large 
auditory capsule is separated, above, from the supraoccipital ( s.o .) by a deep groove 
that contains the “ lateral sinus,” and is partly overlapped by the posterior ( tentorial ) 
edge of the parietal, and lower down by the posterior edge of the squamosal. 
* If the presphenoid (p.s.) were crested, like this part, in front, then we should have a cartilaginous 
septum to the orbits, as in the Bird (see Phil. Trans., 1869, Plate 81, figs. 3, 4, 5) ; in that case the 
cartilaginous crista galli would be an additional crest on the antero-superior edge of this vertical orbital 
septum. 
