174 
MB. W. K. PAEKEB ON THE STBUCTUBE AND 
a position as that of a Teleostean Fish would if a similar segmentation were to take 
place — if the hyo-mandibular were cut away below the opercular knob, and the lower half 
of the bone brought up to the prootic region. The bony metapterygoid process is now 
a mere spur clamping the front of the infrahyomandibular (fig. 8"), which has now 
grown into an elegant hatchet-shaped plate, the sharp bevelled edge of which glides over 
the smooth unossified part of the prootic at some distance outside the prootic and para- 
sphenoidal ossifications. The deep fissure between the continuous metapterygoid and 
the free infrahyomandibular is the persistent upper half of the first cleft (see Plate VIII. 
fig. 8“ as compared with Plate VI. fig. 8, nvpy., Jim.*). A specialization of a lower part 
of this primordial cleft is seen (shown in dotted outline) in the rounded angle between 
the infrahyomandibular and the descending part of the pterygoid (Plate VIII. fig. 8% 
eu., i.hm., pg.) : this is the “Eustachian tube” or passage; and the remainder of the 
cleft has a tympanic function, and is bounded in front by the posterior margin of 
the suspensorium ( qu ., sy., jog.), and behind by the stylo-ceratohyal ( st.li ., c.h.). I have 
already shown that the lower half of this cleft is partly occluded by the symplectic (sy.), 
which now has become completely fused with the quadrate. This stage is excellent for 
showing how the stylo-hyal effects its transit from the infrahyomandibular. First we 
saw this part articulating by a solid condyloid head with the middle of the very elon- 
gated suspensorium behind (Plate V.) ; then, as the upper half of the suspensorium 
became relatively shorter and shorter, thus carrying the stylo-hyal nearer and nearer to 
the skull, we also saw that the condyloid head became shorter, the joint-cavity iost, and 
the fibres of the capsular ligament elongated to form a lengthening fibrous band (Plate 
VII. figs. 1, 3, 5, i.hm, st.h.). 
In this stage (Plate VIII. fig. 8") the “ stylo-hyomandibular ligament” is a long wavy 
band of fibrous tissue, still connecting the stylo-hyal region with the fossa, which was 
originally a “glenoid” facet. 
The two heads of the suspensorium are hidden on the outer side by the supratem- 
poral portion of the temporal bone, and by the large semilunar cartilage, the “ annulus 
tympanicus.” This curved plate does not yet meet at its two ends ; its inner or upper 
edge is thin, and its outer or lower has a thickened and strong selvedge (Plate VIII. fig. 8 a , 
o.t.): the “ extrastapedial” bar (e.st.) is seen through the transparent “membrana tym- 
pani.” 
The structures of the middle ear are best seen from below (fig. 8"). The “ suprasta- 
pedial” (s.st.) is but a feeble ray, and is loosely attached to the periotic where it is pro- 
jecting outward and passing into the metapterygoid root. The “ medio-stapedial” 
( m.st ,) has grown longer, relatively, and is very similar to the columelliform bar of 
the Lizard and Bird. The interstapedial (Plate VII. fig. 14, and Plate VIII. fig. 8 a , 
it.st.) is now broadest behind, but it still retains its heart-shaped outline and is rather 
thin. 
* This space is very large in the adult Bull-frog, and is partly filled up by a membrane-bone, the counter- 
part of which I have not seen in any other- type. 
