DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 215 
of the pterygoids are small, terete, short, parallel, and unusually near together. 
Behind them there is a small foramen, right and left, and still further back, another 
pair of holes, wider apart, near the junction of the basisphenoid with the basioccipital 
(b.s., b.o.). The basis cranii, from the perpendicular ethmoid (over the thick hind rim of 
the hard palate) to the foramen magnum, is a peculiar structure ; in front it is narrow, 
but behind the pterygoid hooks it widens out rapidly into two triangular wings, 
where the ali- and basisphenoid are anchylosed together. The basal plate then narrows 
gently and sinuously up to the middle of the basioccipital region, and then gently 
widens to the end, until it becomes just half the width of the widest sphenoidal part. 
The median part is made into a broadish and rather shallow groove by a thick, ribbed 
part, the rudiment of the hinder part of the tympanic wing of the typical kinds ; these 
thickenings run up to the occipital condyles ; but they do not enlarge the tympanic 
cavity. The inner (and also lower) facet of the glenoidal cavity (gl.c.) turns inwards 
and lies under the foramen ovale—now perfected by a bony hinder bar ;—so that this 
hole is not seen, either laterally (fig. 3) or from below (fig. 2); the longitudinal 
direction taken by this nerve passage is indicated in both figures by a bristle (V*.). 
The inner edge of the squamosal, where it curls under the skull, is sinuous ; it diverges 
from the outer edge of the basis cranii, so as to leave a large space, the hind margin of 
which is finished by the opisthotic region of the auditory capsule (op.). This pyriform 
space, with its wide end behind, is as large as the dilated posterior sphenoidal region 
of the basal and basilateral parts of the skull; it is largely finished by membrane—a 
permanent basilateral fontanelle. This space in the macerated skull occupies nearly 
all the side of the hinder or postorbital part; it is heart-shaped, being filled in 
somewhat in front by the postglenoid wing of the squamosal (see fig. 3). In the 
undisturbed state of the parts where the huge auditory capsule is kept in its place by 
ligamentous tracts, the hinder and outer part of each great fontanelle is filled in by 
that complex, intercalated labyrinth. 
But on the lower face, between the squamosal and the basis cranii, this fontanelle is 
only partially latticed across by the small annulus and the ossccula, which, with the 
drum-membrane, only half hide this great gap. In this under aspect the cochlea (ch/.), 
that part of the vestibule which passes into the tegmen tympani and overarches the 
facial nerve and ossicula, and the part containing the horizontal and semicircular 
canals (h.s.c., p.s.c.), are all well seen. The large, pyriform condyles (o0c.c.) and 
the oval foramen magnum (/fm.) surrounded by the elements of the occipital arch 
(s.0., €.0., b.0.), are shown in this view. 
When the auditory capsules have been loosened out by maceration, and are examined 
separately (figs. 4 and 5; oblique views of the inner and outer side), then we find 
that the capsule has used up and carried with it, in its ossification, a large amount of 
the primary endocranium, not properly belonging to the capsule. Not only is there a 
pterotic and a sphenotic tract of this kind, but, as I have already shown, the small 
alee that originally grew from the basis cranii behind the pituitary region, have been 
