DEVELOPMENT OE THE SKULL OE THE COMMON EOWL. 
793 
true the geographical relations of this little bony territory are to those of the great 
ichthyic “ pterotic ” *. 
The coalescence of the epiotic with the exoccipital is a Reptilian character ; but there 
is a tendency towards that affinity of the periotic centres for each other that we see in 
the Mammalia, inasmuch as the prootic, pterotic, and opisthotic unite together before 
they coalesce with the surrounding bones. Yet this is not very perfectly maintained ; 
for the opisthotic begins to coalesce with the exoccipital about the same time that it 
loses its distinctness from the prootic (see Plate LXXXY. fig. 3, op., e.o.). 
The formation of the tympanic cavity in the Bird is extremely instructive ; the space 
itself is the remnant of the primordial cleft between the first and second poststomal 
arches : this stage illustrates it well. 
In Plate LXXXY. fig. 2 the occipito-otic mass is seen from the outside ; in fig. 3 part 
of the same is shown, but the skull was tilted so as to give an infero-lateral view : it is 
magnified one half more than fig. 2. The floor of the ear-drum, instead of being formed 
primarily of the inferior periotic lip, and secondarily of the tympanic bone, as in the 
Mammal, is here altogether formed by the basitemporal splints. This massive bone 
( b.t .) underlies the eustachian tube (fig. 1, eu .), the “ anterior tympanic recess,” and the 
main part of the chamber. The great wings ( p.r.p .) that grow widely from the 
“lingular” bud of cartilage form a roof and side wall to the “anterior recess;” the 
prootic {pro.) forms the roof of the main chamber, and the exoccipital {e.o.) by its large 
crescentic wing forms the posterior wall. Where the prootic and exoccipital meet above 
the opisthotic {op.), there is formed a very large air-cavity — “ the upper tympanic 
recess ” {u.t.r.) ; the two ear-drums communicate with each other through the diploe that 
lies between these upper recesses. The outer part of the opisthotic is seen on the inner 
side of the drum-cavity, as an oblique bar separating the fenestra ovalis from the 
fenestra rotunda. Above, the opisthotic cochlear bar has coalesced with the exoccipital 
(fig. 3) ; below and in front it is still separate from the prootic. 
The outer notched edge of the prootic (from which the squamosal has been removed) 
is still soft at its selvedge, and a synchondrosial tract is seen running between this bone 
and the crescentic wing of the exoccipital. Halfway between the outer selvedge of the 
prootic and the fenestra ovalis there is seen a thickened rib of bone, which passes directly 
forwards, and is terminated by an oblique, subconcave, oval facet ; this is the point of 
articulation of the inner head or prootic facet of the os quadratum (Plate LXXXY. 
fig. 8, g., pr.f.) ; the other facet of the quadrate is much larger, and articulates with the 
squamosal (fig. 8, sg.f.). This facet for the quadrate lies a little behind the great out- 
growths of the basisphenoid {pr.p.). This articular facet marks the great difference 
between the Fowl’s skull and that of Birds generally; for this is the primordial point 
of attachment, which was once lost (Plate LXXXI. figs. 1, 2, 5 & 5 a , g.), and then 
* Professor Huxley -would refer the epiphysis on the end of the outstanding occipito-periotic mass in the 
Lizard to this element, and he is undoubtedly right ; the same centre keeps cropping up, in one form or another, 
in the Mammalia. 
5 N 
MDCCCLXIX. 
