DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
137 
perforates the capsule, and where it begins its course under the tegmen, which is 
imperfect here in its front part. At some distance below this auditory eave the 
head of the malleus {ml.) is seen in section along the whole course of the 
manubrium on the left side. Under the tympanic cavity and its cartilaginous floor 
the epihyal ( e.hy .) and its accompanying nerves, the chorda tympani and facial trunk 
(VII a ., VII.) are seen cut across, and the hypohyal and thyroid cartilage {h.hy., lx.) 
at the mid-line, below. On the left side the cartilage above the cochlea {chi.) is 
perforated by the facial and auditory nerves (VIII.), and the right side has part of 
the anterior canal {a.s.c.) visible. 
There the tegmen is perfect, and the main part of the incus and its long crus is cut 
through, the manubrium mallei {ml.) appearing beyond and below it. The hyoid and 
its nerves are similar to what is seen on the other side, for the obliquity of this 
section is very slight. 
1 9lh Section (Plate 18, fig. 19).—This partial section is one of the most important 
in the series. The parietal {p.) is seen outside the pterotic or supra-auditorj 7 cartilage 
{s.a.c.), but the squamosal which was seen in the last (fig. 18) is not figured. The 
arch of the anterior, and the ampulla of the horizontal, semicircular canals {a.s.c., h.s.c.) 
are cut through, besides several other spaces in the vestibule and proximal part of the 
cochlea. The wall of the cochlea is cut through in front of the band of cartilage that 
divides the fenestra ovalis from the fenestra rotunda, a band which in the Sauropsida 
is uniformly ossified by the opisthotic centre; in Mammals that centre is much more 
potent, and may, as in the next instance {Talpa), ossify nearly all the proper capsule. 
The auditory nerve and its ganglion {g. cochleare, VIII.) are seen in the meatus 
internus, and externally the stapes {st.) by its base fills the fenestra ovalis. The 
articular part or head of the stapes, which was nearly in view inside the long crus of 
the incus on the right side of the last section (fig. 18) is now cut clean through, and 
the stapedial artery {st.a.) is seen traversing the foot-hole of the stapes, on its way to 
the inside of the maxilla inferior. This section is at the back part of the meatus 
externus, and at the end of the tympanic cavity, so that here the epihyal {e.hy.) is 
seen already confluent with the hind part of the tegmen ; the foramen stylo-mastoideum 
is laid open and the facial nerve (VII.) is seen running downwards behind and outside 
the epihyal, and the cavity of the tympanum {c.ty.) is seen inside, close to the artery. 
The larynx {lx.) is seen below at the mid-line, and the basioccipital floor {b.o.) under 
the hind-brain (C 3 ). 
20 th Section (Plate 18, fig. 20).—Here the supra-auditory cartilage (.s.a.c.) is close 
to the supraoccipital, and the film of bone outside belongs to the squamosal {sq.). 
The anterior and horizontal canals {a.s.c., h.s.c.) are cut across, and the hind part of 
the vestibule at its junction with the cochlea. 
The head of the epihyal is seen finishing the tegmen ( t.ty.), behind, and the narrow 
hind part of the cavum tympani {c.ty.) is shown, here, for the last time. The flat form 
of the parachordals {b.o.), inclosing the notochord, is seen under the hind-brain (C 3 ). 
MDCCCLXXXV. 
T 
