DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 183 
rather thin, somewhat projecting wall-top, right and left of which we see the large 
eribriform plate (cr.p.). Each plate is perfect, is circular in outline—as cartilage, 
and is grooved obliquely, inwards and forwards ; it is also riddled full of holes for 
branches of the olfactory nerve. The top of the presphenoid (p.s.) runs into the hind 
part of the perpendicular ethmoid (p.e.) which is broad behind, and has the thick 
crescentic top of each moiety of the nasal labyrinth pressing against it ; these curious 
curved lobes, which are thick and bulbous against the median cartilage, and thin 
externally, contain the hinder folds of the middle turbinal; they are separated from 
the orbitosphenoids (0.s.) by a narrow chink. These latter tracts are ossified up to 
the thick middle beam (p.s.) ; and the bony deposit is, now, complete for some extent, 
proximally, and then it merely strengthens the fore edge of the tract up to the 
anterior chink. At their inner third, and near the hind margin, these bony centres 
show a small foramen opticum (II.). The hind margin of the orbitosphenoidal stem 
is first convex, and then concave, and this sinuous line, the front boundary of the 
sphenoidal fissure (V1.) is a good height above, and not behind, the fore edge of the 
alisphenoid ; it crosses over the foramen rotundum (V*.) 
The rest of the alisphenoid (a/.s.) is seen, away from the eye, behind and below the 
orbitosphenoid (0.s.); the basisphenoid (b.s.) is, on this upper surface, of less lateral 
extent than below ; it only passes for a small extent beyond its own boundary line. 
The synchondrosis is large, and in it is seen the seat of the sella turcica (py.) and the 
low transverse postpituitary wall (p.cl.). The middle fissure, right and left, between 
the synchondrosis and the cochlez (ch/.) is wide; the “helix” projects upwards, but 
is not so well seen as below; the meatus internus (VIL, VIII.) is wide and large. 
The fore margin of the auditory capsules runs almost transversely across the lower 
part of the base of the hind skull, and the outer two-thirds of this edge forms the 
hinder boundary to the great infero-lateral fontanelle. The separateness of the 
capsules from the chondrocranium, proper, is very perfect, especially on the anterior 
and inner side; but above, it is more apparent than real, for there the great sinus 
canal (/.s.) seems to part the crest from the tract containing the semicircular canals. 
The whole arch of the anterior canal (a.s.c.) shows its convexity here, and under the 
archway there is the large recess for the ‘“ flocculus” (f.r.). The lower part of the 
occipital arch binds, sinuously, against the two huge capsules ; in this view we see 
most of the exoccipitals (¢.0.) and all the basioccipital (b.0.), the fore edge of which 
nearly reaches the post-clinoid wall (p.cl.). The cartilage is thick; it is perforated 
by the 12th nerve behind the posterior fissure, and somewhat notched by the 9th and 
10th nerves (IX., X.), in the margin, behind that fissure. 
Ninth Stage (continued).—Visceral arches of a Young Mole ; 13 inch long. 
I shall finish my description of the skull at this stage by an account of the inferior 
arches of a somewhat smaller specimen than the one whose main skull has just been 
treated of. The front fourth, and the hinder half, of the mandible are shown from the 
