308 MR. W. K. PARKER ON THE STRUCTURE AND 
portio dura (7*), and mesiad of the jugular vein the glossopharyngeal nerve (8*) is cut 
through; outside the nerve, and below the stylohyal (sé./.), a part of the exoccipital 
(¢.0.) appears. A similar section to the other side is also shown as a front view (fig. 8) ; 
and here the relation of the portio dura (7*) to the hyoid arch is well seen. This figure 
does not show the incus, which is removed to display a new segment (/./y.) that has 
arisen, the counterpart of the Sauropsidian “ infrastapedial ” (H.) and of the so-called 
“ stylohyal” of the Fish (Cuv.); here it will be called the ‘‘interhyal ” (P.), as it really 
wants a proper name. In the second stage (Plate XXX. fig. 6) I have displayed the 
relation of the segmenting second arch to the very Batrachian stapes (see “‘Frog’s Skull,” 
Plate vu. figs. 12-16, sf.); and on the same Plate (fig. 8) I have put for comparison 
the state of things in the third stage; both are front views, and the apex of the stapes 
is towards the eye. ‘The bigeminal papulie on the younger stapes are now connected by 
a bridge of cartilage, and the lateral dimples are the foot-hole of the little stirrup. 
The round head of the short crus of the incus is seen articulating with the tegmen 
tympani (Plate XXX. fig. 8, ¢.ty., 7.); and below and behind it is the clavate head of the 
stylohyal, which is ready to coalesce with the periotic cartilage at the junction of the 
epiotic and opisthotic regions (see Plate XXXVL. fig. 2, @., s¢.., op., ep.). The descent 
of the dislocated hinder half of the shoulder of this second arch is not so great as in the 
Osseous Fish (‘*Salmon’s Skull,” Plate vr. fig. 2), but it is considerable; and this is a 
true third stage, as may be seen by comparing fig. 9, Ay., in Plate XXIX. with figs. 
6 & 8, Plate XXX. ‘The binding, intervening band of new indifferent tissue which 
has grown in the gap of these divided parts has acquired a hardening nucleus of new 
cartilage, exactly as we see it in Ganoid and Osseous Fishes, e. g. Accipenser, Anguilla, 
Salmo. The portio dura (7*) is seen passing down its aqueduct behind these segments, 
and the upturned, inbent, long crus or neck of the rib-like bar ends in an elegant sucker- 
shaped disk, its capitulum or apex*. 
On the same Plate the postoral arches of the third stage are shown in a side view 
(Plate XXX. fig. 9); and a comparison of the undivided mandibular bar with the displaced 
fragments of the hyoid will make things plain to the mind. ‘The apices of the two 
bars come very near together; but whilst the first hooks backwards, downwards, and 
inwards, it does not graft itself upon the auditory sac; nor does its shoulder send back- 
wards a secondary pedicle so distinct as the ** short crus of the incus.” Moreover the shaft 
of the bar on the first arch keeps on its way normally, as in the early embryo; but in the 
second arch this part has been segmented off, and displaced backwards and downwards, 
catching in its descent at the xeck and head of the arch, but travelling still further in 
more advanced stages, until it rests and combines with the postero-inferior angle of the 
auditory mass. In the new web which grows between the two segments comes the 
secondary “ interhyal” segment; this, however, loses all its first relations, and finally 
* In both these figures, put together for comparison, the parts of the second arch are coloured, and those of 
the auditory capsule are plain, for the sake of distinction, that the eye may learn to separate the “stapes” 
from the segments of the hyoid arch, 
