DEVELOPMENT OF THE SKULL IN THE MAMMALIA, 229 
the opisthotic region grows into a distinct mastoid process. The fenestra ovalis (/80.) 
is seen inside the canal for the facial nerve, and the large fenestra rotunda (/f.7.) is 
reniform, being notched somewhat in front. A considerable tract of the bony capsule 
is seen behind the squamosal (figs. 3, 4); the canals (c.s.c¢., h.s.c., p.s.c.) mark this 
part, but the anterior canal is best seen on the inside (fig. 5), in front of the recess 
for the “ flocculus (/l.7.).” Behind this moderate hollow the thickened bone contains 
the common sinus of the anterior and posterior canals. The large meatus internus 
(VIL, VIIL.) is oblique, going backwards and downwards ; the passage for the facial 
nerve, in front, is well marked. The occipital arch has its elements still separated by 
considerable tracts of cartilage; the basioccipital (b.0.) is a sinuous, transversely 
polygonal plate, sharply notched in front, where the notochord ran, and having a 
concave margin behind, at the foramen magnum. ‘The supraoccipital (s.0.) is a large 
shield of bone, thickened and convex along its middle, and arching over the foramen 
magnum by its lower edge. The exoccipitals (e.0.) form a very small paroccipital 
ridge behind the mastoid process ; the foramen condyloideum (XII.), close behind and 
within the posterior lacerated foramen (IX., X.), is large. The whole arch is sinall 
relatively to the rest of the skull, and this is Eutherian in this respect. 
MECKEL’s cartilage (figs. 3, 6, mk.) is being lost in the mandible in front ; where it 
has become free behind, tracing it backwards and upwards, it is still quite thick and 
is endosteally ossified, continuously with its proximal part, the malleus (m/.). The 
primary ossification (Plate 32, fig. 7, ml.) is ectosteal, and now, on the inside, this is 
roughly and imperfectly subdivided into three bony laths, binding the front of the 
head of the malleus, which is now well ossified throughout, the endosteal tract seen 
in the early state (Plate 32, fig. 7, m/.) having used up all the cartilage except the 
selliform condyle. 
Here we have the counterpart, first, of the endosteal or inner articulare of the 
Sauropsida ; and then the outer articulare, the supra-angulare, and the angulare of 
the endocranial mandible, in a state of imperfect differentiation. 
Below the condyle the malleus projects towards the incus (7.), the head is then bent 
on itself, growing obliquely forwards ; it gives off two processes, manifestly equal to 
the internal and posterior angular processes of the Bird; the latter is represented 
here by a rounded knob (ay.p.), and the former by the long straight, slender 
manubrium (mb.). On the outside the malleus is strongly tied by the anterior crus 
of the annulus (figs. 7 and 8, a.ty.); behind that bar the malleus grows into a crescentic 
foliaceous plate and is concentrically grooved to a less degree in front of the condyle, 
and much more in front of the foliaceous outgrowth. All these things admit of no 
Teleological interpretation, but show that the hinder third of the Sauropsidan type of 
mandible is here aborting itself, so to speak, into an Eutherian malleus. 
The incus (fig. 6, 7.) is well formed ; the short crus (s.c.7.) straight and conical, and 
the long crus (/.c.7.) is short, capped with an orbicular facet where it turns inwards 
to articulate with the stapes. 
