1S4 
ME. W. Iv. PAEKEE ON THE STEUCTUEE AND 
compared with the embryonic skull of the Teleostean (Huxley, ‘ Croonian Lecture,’ p. 29, 
tig. 8, and ‘ Elem. Comp. Anat.’ p. 185, tig - . 72), it will be seen that the hyo-mandibular 
runs almost directly forwards , and that the boss to which the “ opercular ” is attached 
grows out a little distance below the posterior hyo-mandibular fork. Now the segmen- 
tation of the hyo-mandibular in the Frog takes place immediately below this boss, leaving 
it free ; and from the first it has a direction forwards, which is only intensified and not 
altered in the adult. The “ medio-stapedial ” ( m.st .) has now all the appearance of the 
osseous auditory “columella” of the Lizard; it is twisted and curved somewhat, is very 
slender at first, and then bulges behind ; it is bevelled at its end, towards the skull, and is 
slightly unossified at its edge behind. The bevelled inner face of the “medio-stapedial” 
articulates with the lower half of the outer face of the “ interstapedial,” the segment 
which was taken from its base. This intercalary piece, the undoubted homologue of the 
mammalian “ os orbiculare,” has now become a solid wedge of hyaline cartilage, and has 
found lodgement in the anterior part of the deep “stapedial fossa.” The “interstape- 
dial ” is pointed in front, has a convex upper, and concave posterior and lower margins ; 
it is thick and solid, especially behind, where it articulates with the stapedial plate. The 
homology of the “ interstapedial ” is with the condyle of the “ posterior hyo-mandibular 
fork” of the Osseous Fish ; its function is to connect the true “stapedial” (or auditory) 
segment with the medio-stapedial hyoid element. The periotic element, “ stapedial ” ( st .), 
is elegantly elliptical in shape ; but the anterior margin is shortened, where it fits to the 
subconcave face of the “ interstapedial” (Plate VIII. fig. 9, st., i.st .) ; it is gently concave 
on the inner face, and is very thick and convex on the outer (Plate VII. fig. 15, st.) ; like 
the other parts of the middle ear, with the exception of the medio-stapedial, it is wholly 
unossified. It is attached to the edges of the fenestral fossa by a delicate band of fibrous 
tissue (see Plate VIII. fig. 10, where it is seen from within) ; but much of the inner face 
is in immediate contact with the cavity of the vestibule. 
The “fenestral or stapedial fossa” (Plate VII. fig. 16, st.f.) is beautifully egg-shaped 
and of considerable depth (see section, fig. 15) ; behind, the exoccipital ( e.o .) keeps at 
some distance from its rim; but in front the prootic {pro.) sends a small wedge of bone 
into its fundus. The “fenestra ovalis ” ( fs.o .) takes up the postero-inferior third of 
the fundus; it is reniform, with the concave edge looking obliquely forwards and 
upwards; it nowhere reaches the edge of the pit; the main otoconial mass (ot.) can 
be seen through the “fenestra.” This operculated cleft in the “periotic” wall was 
once the size of the stapedial plate ; the floor of the pit has grown, whilst the stapedial 
plate has been thickening, as the creature has become full-sized. The “ stylo-hyal ” (st.h.) 
has its permanent attachment a short distance below the rim of the fossa at its interior 
third (Plate VII. fig. 15, Plate X. fig 9, st.h.). 
The “ stylo-cerato-hyal ” band of cartilage has changed but little since the Frog was 
two or three months old ; it has not become osseous (Plate IX. fig. 3, st.h. and 
Plate X. fig. 2). The “hypohyal ” region ( liy.h .) retains the lobe, both before and be- 
hind, which was seen in the Tadpole (Plate V. fig. 2) : these lobes have now become 
