DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
207 
Then there comes, behind and outside the posterior sphenoid, a very large space, in 
the dissected endocranium, which in the perfect skull (fig. 2) is only partly filled in 
by the squamosal ( sq.). This was the case very remarkably even in the skull of the 
embryo (fig. 7), but the auditory capsule has not grown so much, since, as the rest of 
the skull and snout (figs. 3, 4); it scants very much towards the great semicircular 
space developed for its reception. The small perforated ala of cartilage seen in the 
embryo growing right and left, from the basisphenoidal region (Plate 29, fig. 7 ; and 
Plate 30, fig. 18), and coiling round the front of the cochlea—helping to complete the 
setting—is now absent; it has evidently been used up in the ossification of the 
cochlea (chi.); it now exists as the toothed rim of its semicircular fore part. There 
is still some cartilage between the prootic plate ( pr.o'.) and the main part of the 
capsule ; and also behind the bony top of the epihyal (,e.hy .), as well as on the inner- 
face, above (Plate 31, fig. 11). The coils of the cochlea (chi.), the fenestra ovalis and 
fenestra rotunda (fs.o., f.r.), and the great fissure widening for the exit of the 9th 
and 10th nerves (IX., X.) are clearly seen in this lower view. 
The wide basioccipital (b.o.) with its semicircular notch in front of the foramen 
magnum; the large reniform condyles (oc.c.) ; the exoccipitals ( e.o.) separated by a 
large tract of cartilage from the base, and that tract perforated by the 12th nerve 
(XII.), are also shown. There is scarcely any paroccipital ridge to the exoccipital 
bone, which, above, is separated by a definite tract of cartilage from the large, shield- 
shaped supraoccipital (s.o.). 
In the upper view (Plate 20, fig. 3) the nasal labyrinth is complete (al.n., al.sp., 
al.c.), and ends above, as below, in a rounded lobe right and left. The fluted top part 
is very uniform up to the proper olfactory region ; that region is ossifying, rapidly, in 
its middle. The grooved top, formed by the septum (s.n.) does not end in a car¬ 
tilaginous crista galli ; from the median, notched part the margin of the nasal roof is 
first concave, and this swells into the lateral, terminal convexity. 
The great perforated floor, or lamina cribrosa (cr.p.), has, outside, a sublobate edge ; 
the rest is evenly semicircular; more than half of the plate, with the top of the 
perpendicular ethmoid, is ossified. In this curiously scanted skull there is a con¬ 
siderable membranous space between the cribriform plate and the orbitosphenoid ; 
these tracts (o.s.) are remarkable in several ways, and are almost the least normal of 
any I have yet seen in the Eutheria. The top part is still fan-shaped, but the whole 
of the large posterior band (see fig. 7, o.s.) is gone. The sharp angle left is now 
ossified separately (o.s'.), and the stem (o.s.) is also bony; it does not reach more 
than two-thirds of the way to the upper margin of the wing. The lower end is 
pedate, and is beginning to form a suture with the base (p.s.) ; it is then reduced to 
a mere rod, which curves forwards and backwards twice, becoming flat, but still narrow, 
above. 
There is no optic foramen, as in Marsupials; but in front of the main bar, below its 
middle, there is, in a notch, a separate spicule of bone (o.s".), a third bony centre to 
