DEVELOPMENT OF*THE SKULL IN THE MAMMALIA. 199 
it passes into the gently arched snout (a/./.). The basal cartilage behind the cribriform 
plate and uasal labyrinth is thick, but from being high and narrow, becomes depressed 
and broad; thus the middle ethmoidal region passes into the short presphenoidal, 
and that into the fore part of the basisphenoidal territory ().s.). That region is per- 
forated and partly ossified (see Plate 30, fig. 18, b.s.). Here we meet, again, with the 
open pituitary space which we found in the Hedgehog (Plate 17, figs. 1, 2, py.), but did 
not find in the Mole (Plate 25, figs. 2, 3). Behind that passage the basal tract thins 
out, becomes gently concave, thickens again, is again ossified, partly, as the 
basioccipital (b.0.), and then ends, with a narrow cartilaginous selvedge, at the foramen 
magnum. The primary fontanelle is very large, for the band of cartilage connecting 
the great orbitosphenoidal wing (0.s’.) with the equally large supra-auditory tract 
(s.a.¢.) 1s narrow. 
The upper orbitosphenoidal expansion is confluent with the nasal wall in front; it is 
sinuous, notched in a rugged manner, above, and its hind margin, both above and below 
the narrow posterior band, is concave ; there is no optic foramen through its stem. The 
optic nerve passes through the great sphenoidal fissure, in company with the orbital 
nerves, as in Marsupials. The skull is unprotected by cartilage tor a very large space 
both above and below the narrow band ; for that band, dipping a little, bounds a huge 
space below of an irregularly oval shape, with its long diameter—which reaches from 
the cribriform plate to the meatus auditorius internus (VIL, VIII.) —two-fifths the 
length of the whole cranial cavity. Below, this space is partly occupied, in front by 
the alisphenoid (u/.s.), and behind by the cochlea (ch/.). The alisphenoid (see also 
Plate 30, fig. 18, a/.s.) is a small ear-shaped tract of cartilage, notched on its hinder 
margin near its upper angle for the 38rd branch of the trigeminal nerve (V*.), and 
lying quite outside the plane of the orbitosphenoid. 
Thus the optic, 3rd, 4th, Ist and 2nd branches of the 5th, and the 6th 
nerves all pass through the gaping sphenoidal fissure. Some distance behind the 
proper alisphenoid the basis cranii becomes alate, and that small rounded wing is per- 
forated and has its hinder margin adapted to the apex of the cochlea, on its inner side. 
The posterior orbitosphenoidal band (0.s’.) runs upwards and forms a sinuous ribbed 
edge to the supra-auditory plate (s.a.c.), the high upper margin of which has a convex 
outline, and passes into the supraoccipital region (s.0.), already ossifying. 
That spheno-pterotic tract (s.u.c.) is as broad as the inner face of the huge, genicu- 
late auditory capsule (chl.), its hinder half, and the capsule, form the very perfect 
semioval margin to the hinder part of the great lateral fontanelle. The archway 
formed by the upper cartilaginous wall (s.a.c¢., s.0.) for the setting of the oblique upper 
half of the auditory capsule, is much larger than is needed for that part; the whole 
margin is occupied by the large, and accurately semicircular “ lateral sinus” (/.s.). 
There are three arched and convex tracts to be seen in the postero-superior part of 
the auditory capsule; the middle tract is nearest to, the hinder tract furthest from, 
the eye. The foremost arched convexity contains the anterior canal (a.s.c.), and 
