296 PROF. W. K. PARKER ON THE STRUCTURE AND 



It is bound in and covered by strong outer plates of bone (Pis. LXIX. & LXX., q, 

 sq, q.j), and only shows itself between these tracts below (PI. 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 (PI. 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 {tyvqMMc) 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 [q.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.])., 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 (PI. LXX. fig. 7), runs obliquely down- 

 wards and backwards from the front angle above to the fore edge of the articular 

 condyle {qx). The upper edge is also developed into a rounded balk of bony sub- 

 stance; thus the postero-extemal 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 {e.st) 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" {ar) ; this is the lowermost and the hiudermost 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 (PI. 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 (PI. LXIX. figs. 9-11, PI. LXX. fig. 1, and 

 PI. LXXI. 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 



