DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 3 
ala, on its inner edge, arises a recurrent cartilage. Together, in this view, these 
look like a lanceolate leaf, with convex halves, like that of Magnolia grandiflora. 
The walls are developed into the inferior turbinals (7.tb. ; see also Plate 8, fig. 9, in 
Bradypus) ; the inner part of the floor is developed behind them. Between the 
halves of the nasal floor the middle vomer (v.) is seen ; it is short, notched behind, 
carinate, and has its fore end, which is blunt, hidden by the recurrent cartilages ; 
behind the vomer, the widening basis cranii is seen where the vertical ethmoid passes 
into the presphenoid (p.c., p.s.), This latter region (fig. 5) is still unossified ; it 
passes insensibly into the perpendicular ethmoid in front, and on each side grows into 
a large wing, the orbitosphenoid (0.s.), which is perforated at its base by the optic 
nerve (II.). In this stage the cartilage ascends and becomes a large plate, continuous 
by its fore corner with the margin of the rhinencephalic fossa, floored by the cribriform 
plate (cr.p.), Below that junction there is a crescentic fenestra between the two 
regions, then the orbitosphenoid rises inside the orbital plate of the frontal (f), 
and then dips again heneath the hind lobe of that bony plate, Across its widest part 
the orbitosphenoid is nearly equal to the great internasal partition. After a little 
irregularity of margin the cartilage runs on, rising upwards again as a narrow band, 
and then loses itself in the wide supratemporal lamina, which, in its turn, passes into 
the supraoccipital (s.a.¢., s.o.); the upper edge of the supratemporal tract is crenate, 
and so is its front margin, where it forms the postero-superior margin of the huge 
oval lateral fontanelle. That space is as wide as the auditory capsule, and wider than 
the orbitosphenoid ; it is covered in, externally, all but its anterior and upper margin, 
by the alisphenoid (a/.s.) below, and the squamosal (sq.), still further outwards, above. 
That peculiarly Mammalian character, the out-thrust of the alisphenoid, is well 
seen here ; this “ala” is now ossified as a thick bilobate mass, which leans backwards 
on to the tilted auditory capsule; it is still separate from the basisphenoid (b.s.) ; 
this bony deposit does not reach the low, transverse postclinoid wall, At present, I 
see no proper foramina in the alisphenoid (qa/.s.), but the branches of the Sth nerve 
(V'., V°.) evidently pass through mere fissures. Over the notched top of the ali- 
sphenoid we see part of the squamosal (sq.), and the 3rd branch of the 5th nerve 
(V*.) escapes, at present between that bone and the alisphenoid (@/.s.), in the chink 
between its lobes, whilst the Ist and 2nd branches (V"*.) emerge through the sphenoidal 
fissure ; this bone is very rudimentary at present. 
The rest of the endocranium, proper, is well distinguished from the auditory capsule, 
which is tilted back at an acute angle to the basis cranii. The whole capsule is one- 
third larger in outline than the fenestra in front of it ; the cochlear portion is jammed 
in between the basal beam and the infero-lateral walls (see also Plate 8, fig. 1), whilst 
the upper and hinder margin is ribbed all round by the thickening outside the anterior 
and posterior canals (a.s.c., p.s.c.) ; this thickening is mereased and turned downwards 
where the two canals are fused into one “sinus.” Behind the ampullar region of the 
anterior canal there is a notable aperture, seemingly for the facial nerve (VIT.), but 
