296 PROF. W. K. PARKER ON THE STRUCTURE AND 
It is bound in and covered by strong outer plates of bone (Pls. LXIX. & LXX., ¢, 
sq, q-j), and only shows itself between these tracts below (Pl. LXX. fig. 1); the inner 
(or lower) face of the quadrate is seen to be very broad and somewhat concave. When 
cleared of its surroundings, and its outer wall removed (Pl. LXX. fig. 7), this bone 
shows large pneumatic cavities that traverse every part where the thickness is sufficient ; 
on the inner face the table is very imperfect, and without any paring away shows the 
large air-cavities; they open freely into the first (tympanic) cleft. The middle third 
of its hind margin is notched, so as to form a large circular opening, finished behind 
by the hyoid cartilages; through this passage the columella escapes to lie on the hollow 
outer face of the quadrate. This bone is roughly four-sided, but the upper edge, or 
otic process, is extended fore and aft, and these rounded angles are not yet ossified. 
The hind margin is generally concave, but has the large notch in it; the lower is 
sinuous, ending behind in the large cylindroidal condyle (¢.c), and in front runs to 
the end of the “orbital process,” which is not yet ossified at the forked end. The 
ascending and pterygoid spurs (a.p., pg.c) are now very short. Above these, on the fore 
edge, there is a toothed process on an outline which is gently concave. A thick rib of 
bone, partly cut away in the specimen figured (Pl. LXX. fig. 7), runs obliquely down- 
wards and backwards from the front angle above to the fore edge of the articular 
condyle (q.c). The upper edge is also developed into a rounded balk of bony sub- 
stance; thus the postero-external face of the bone forms a large shallow crescentic 
space, over which the tight tympanic membrane is drawn, and under which, at the 
middle of its upper part, the extrastapedial end of the columella (¢.s¢) projects; this is 
analogous to the “manubrium mallei,” but its homology with it is doubtful. A large 
air-cell runs inside the front oblique ridge, and a lesser cavity is seen below the hind 
notch; the pneumatic opening of this lesser cavity is halfway down the solid part under 
the notch. From that aperture there proceeds a membranous tube, which forms a 
communication with a similar aperture on the top of the articular region of the 
mandible close behind the joint ; this tube is the “siphonium” (Nitzsch). In the figure 
a bristle is shown running through the upper space; below there is a large bilobate 
cavity in the “os articulare” (a); this is the lowermost and the hindermost part of the 
extensive tympano-Eustachian labyrinth, formed by specialization of the “ first visceral 
cleft.” 
In front of that hollow bony centre the mandible is a soft and terete rod, coalesced 
with its fellow at the chin (Pl. LXX. figs. 12, 13, Pl. LXXI. figs. 1, 2, 5, 7, mk). In 
the coronoid region the rudimentary coronoid tract of cartilage is still seen facing the 
mandible, where it would chafe against the huge wing of the pterygoid bone, that bone 
having also a facing of pterygoid cartilage (Pl. LXIX. figs. 9-11, Pl. LXX. fig. 1, and 
Pl. LXXL. fig. 7, cr.c, pg.c). 
The upper elements of the hyoidal arch (Pl. LXIX. fig. 4, Pl. LXX. fig. 7, and 
Pl. LXXI. fig. 7) are now seen as distinct and, for the most part, reduced and arrested 
