DEVELOPMENT OP THE SKULL IN THE MAMMALIA. 
97 
against the cochlese (chi.), and behind; there it has thick-ribbed edges, which make 
the middle part of the bone concave; this is like what is seen in the Unau (Plate 9, 
fig. 1) ; this is shown still more where these ridges are functional, as in the Anteater 
(Plate 10, fig. l)/“ 
The oval basioccipital (b.o.) is notched a little where it forms the front boundary of 
the foramen magnum ( fan .); on each side of that end the large occipital condyles 
are seen (oc.c.); their direction is infero-posterior. The exoccipitals (e.o.) run well 
outwards to the grooved and rough opisthotic (op.), which is wedged in between the 
squamosal and the gently convex paroccipital ridge. 
The large hypoglossal (condyloid) foramen (XII.) is partly finished inside by 
cartilage, and the exoccipital bone only partially surrounds that nerve and the 
9th and 10th (IX., X.) at their exit; the cartilage here is half as broad as the bone 
outside it. The bony tracts seen in the distance behind the foramen magnum are not 
parts of the supraoccipital but an extension of the lateral bones (see also fig. 9, e.o., 
s.o.) The moderately broad annulus tympanicus has been left in situ on the left 
side (Plate 12, fig. 6, a.ty.); on the right side of the skull this bone and the 
ossicules were removed (see also Plate 13, fig. 1). This imperfect ring has doubled 
its breadth since the last stage (Plate 11, fig. 8), but it fails to run along the meatus 
externus, and leaves the large membrana tympani exposed in this view. 
The well and roughly ossified auditory capsule (Plate 12, fig. 6 ; and Plate 13, 
fig. 1, chi., op.) runs obliquely inwards and forwards, warty, as it were, and riddled 
with holes and passages. The facial nerve (VII.) runs through a bony canal, and 
appears on the roof of the tegmen tympani to escape again through the stylo¬ 
mastoid foramen, which is bridged over by the small confluent epihyal (e.hy.) (see 
also Plate 11, fig. 8, e.hy., VII.), which has become confluent at its once free end with 
the highly ossified capsule, behind the obliquely seen fenestra rotunda (fr.). Another 
small bridge of bone at right angles with the inner end of the epihyal bridge 
separates the fenestra rotunda from the fenestra ovalis (fs.o.). In front, the ossified 
wall of the cochlea (chi.) fits into digitations of the alisphenoid (al.s.) by knobs of bone ; 
and, behind the stylomastoid foramen (VII.), the opisthotic region, is divided into 
two rough convexities by a considerable groove. On the inner face, where the cochlear 
and opisthotic regions unite, there the bony mass is notched twice, to let out the 9th 
and 10th cranial nerves (IX., X.). 
The pyriform shape is well seen in the upper view of the skull (Plate 12, fig. 7). 
The snout (al.n.) is twice as much seen here as in the lower view (fig. 6). The only 
additional parts that come into view as compared with the last stage (fig. 1) are 
the squamosals (scp) that can be just seen flanking the parietals (p.). The pre- 
maxillaries ( px.) are seen in front, and the maxillaries (mx.) can also be seen flanking 
the frontals (f.) ; the partly ossified supraoccipital (s.o.) finishes the structure behind, 
The still-notched, but improved, nasals (n.) run well back between the frontals, 
* This character reappears in normal tooth-bearing Mammals, e.y, in the Marmot ( Arctomys monax ). 
MDCCCLXXXV. O 
