DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
221 
which did not ossify, but was absorbed. Each of these bony plates is widest above 
and narrowest in the middle, and the large optic foramen (II.) is well in the middle of 
the proximal part. When fully seen from above (fig. 5, o.s .) the hind margin has two 
emarginations, one large, in the middle, and one small, near the base ; the projection 
between these notches has, on the right side, a small additional nucleus of bone ( o.s a .). 
The posterior sphenoid is at this early stage a single bone, for the base and the alee 
( b.s., cd.s.) have already lai'gely become confluent; the lower surface (fig. 4) is much 
larger than the upper (fig. 5), this is mainly due to the great tympanic development 
in the base; but the alee are partly overlaid both by the orbitosphenoids and the 
cochleee (o.s., chi.) ; the alisphenoids are most remarkably out-thrust in these and 
other normal Insectivores. Half the cartilage between the basisphenoid and the 
presphenoidal tract of bone (fig. 4, b.s., p.s.) belongs to the former and half to the 
latter; also, behind, there is much cartilage between the basioccipital (b.o.) and the 
basisphenoid. 
There the basal beam has doubled its width ; but that is only two-fifths of the 
whole width of the basisphenoid, which has two crescentic wings, shell-like out¬ 
growths, placed back to back, with their hollow face outside. In front, these tympanic 
wings only reach half way along the basisphenoid ; behind, they overlap the basi¬ 
occipital, so that they are retral in relation to their origin. The whole inferior sur¬ 
face of the posterior sphenoid is broken up into ridges, grooves, and holes. On each 
side, in front, there is a rough ridge parallel with the basal cartilage and bone ; these 
are for the fixation of the basicranial flanges of the palatines and pterygoids. Outside 
these, separated from them by a fossa, there is, right and left, a triangular wing, the 
external pterygoid process (e.pg.) ; the hollow is the pterygoid fossa; these wings 
spread further out than the tympanic wings, behind, and they resemble them in form. 
Outside the front of the curved tympanic wing there is a rough foramen (f ovale, V 3 .) 
which on the inside (fig. 5) is a mere notch. Outside and in front of this is the hinder 
opening of the alisphenoidal canal (fig. 4, al.s.c.), and on the same oblique line, looking 
outwards and forwards, is a semioval fossa, its truncated end being at the edge of the 
great wing (al.s.). On the inside this is not seen, and the bony wing has a generally 
concave face in front of and outside the foramen ovale (V 3 .). The posterior clinoid 
wall (fig. 5, p.cl.) is low and broad; there is, however, a very definite sellar 
depression. 
The auditory capsules—here artificially squeezed outwards to display their parts— 
are large and pyriform ; they are quite normal in having two subequal osseous centres 
—the prootic and opisthotic ( pr.o., op.). The latter, on the lov T er face (fig. 4), 
embrace the base of the cochlea (chi.), and the two are fast approaching each other 
over the first coil. The prootic (pr.o.) is creeping backwards, and (fig. 5) along the 
front edge of the capsule. 
The opisthotic is seen from below (fig. 4, op.) creeping round as an unciform shell, 
behind and under the cochlea and vestibule; it has already formed the familiar inter- 
