DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
51 
is seen passing upwards and baokwards along the middle of this hinder membranous 
tract. 
We shall see, afterwards (Plate 9), how completely this space is filled in by the 
frontal and squamosal; the latter is now a large ear-shaped scale of bone with a thick 
horizontal part, capped with the glenoid facet (see fig. 1). The temporal fossa, 
widely opening into the orbit, is but slightly walled in externally by the projecting 
articular part, and its small zygomatic process. The inner plate of the squamosal 
reaches further forwards than the outer; behind, the bone ends in a round post¬ 
temporal lobe, which overlaps the massive ear-capsule up to the junction of the anterior 
and posterior semicircular canals (a.s.c., jo.s.c.). The occipital plane is very wide and 
transverse (see fig. 4) ; here, in the side view, little save the edge of the arch, with 
its ossifications ( s.o ., e.o.), and the condyle ( oc.c .), are seen. At present, the mandibular 
ramus ( d .) is very slender, not much stouter than that of Tatusia ; the absence of 
teeth ntakes the fore part narrow (Plate 8, fig. 3 ; and Plate 9, fig. 9). But the coro- 
noid, articular, and angular processes are large, and largely cartilaginous ; the first 
of these is long and distinct, the next solid and wide, the next a large rounded 
lobe. 
The annulus tympanicus (a ty.), although narrow and simple, is quite ossified 
already; Meckel’s cartilage ( mk .) is seen in front of it ; the head and manubrium of 
the malleus ( m.ml .), are seen between its obliquely placed crura ; and behind the 
head of the malleus the cartilaginous junction of the ear-capsule with the epihyal 
is seen, behind which the facial nerve (VII.) is emerging. The hyoid arch is short, 
and the segmentation below' what is normal even for a branchial arch ; not only 
is the ceratohyal ( c.hy .) not re-segmented, but it is continuous with the epihyal. 
The hypohyal (h.hy ) is a longish curved tongue-shaped piece, pointed in front, where 
it nearly meets its fellow, and articulated loosely with the fore parts of the U-shaped 
basal piece ( b.h.br .), for the thyrohyals ( t.hy .) are not segmented off from it. The 
only bony deposit is a sheath to the main part of the ceratohyal. 
The end view (Plate 9, fig. 4) is roughly oval, the long diameter being across, and 
the lower margin least convex and with several sinuosities. The upper view (fig. 2), 
which fails to show the full size of the parietals (p~), is corrected by the side and end 
views (figs. 3 and 4). Here they are seen with gently sinuous margins, and meeting 
at a right angle to overlap the upper margin of the occipital arch and the hind part 
of the auditory capsules. Below these, right and left, just the thin slightly inturned 
hind edge of the squamosal (sq.) comes into view, binding over the place where, within, 
the ampullae of the anterior and horizontal canals lie. For so early an embryo the 
supraoccipital (s.o.) is a very large osseous centre ; it has, recently, become one by the 
union in most of its lower half of two centres of bony deposit; much of the suture is 
seen above, and a little of it below. The part next the two parietals is nearly a right- 
angled triangle; on each side the lower angles lie on the auditory capsules. Below 
them the bone, keeping to its own proper chondrocranial field, has a short concave 
h 2 
