292 PROF. W. K. PARKER ON THE STRUCTURE AND 
canal for the internal carotid artery (7.c), which reappears in the basisphenoid, and ends 
in the bottom of the pituitary cup (Pl. LXX. fig. 4, 7.c; here the basisphenoidal part 
of the canal is drawn as having a bristle passed through it). 
The two large tracts of the exoccipital that unite, respectively, with the basioccipital, 
superoccipital, and opisthotic (op) are not faced with more than a trace of cementing 
cartilage; for large cavities have been already formed. But the tympano-Eustachian 
labyrinth communicates with all these posterior cranial and auditory bones; the fore- 
most of these spaces, on the opisthotic margin, is seen in the partly disarticulated skull 
(Pl. LXX. fig. 3, ¢.0). The inner wall of the exoccipital (Pl. LXIX. figs. 7, 8, ¢.0) 
is a couyex tract, narrow in the middle and dilated above and below; its posterior 
margin is sinuous, and its front margin is bevelled and oblique behind the large 
occipito-auditory chink, which below lets out the ninth and tenth nerves (1x, x). The 
opisthotic (op) bulges towards its fore margin, and is ankylosed to the exoccipital, now, 
in front of the chink, and projects inwards asa shell of bone, which is angulate in 
front. Below (Pl. LXIX. fig. 8, op) it is narrower, and forms a complete loop of bone 
round the fenestra rotunda (/.r) and divides it from the f. ovalis ( fs.0). 
The supraoccipital is entirely on the roof of the skull (Pls. LXIX. and LXX. s.o); it 
also is compound, now, for it has coalesced with the right and left epiotics (ep). It is 
rhomboidal in form, and its hinder projection is separated by a wedge of cartilage from 
the foramen magnum ; cartilage also can be seen within it behind (Pl. LXIX. figs. 7, 8, 
s.o), and also above over its junction with the epiotics (Pl. LXX. fig. 6, s.o, ep). It is 
concave in the middle and scooped right and left in its thick exposed hind part; but in 
front, where it is covered by the parietals (Pl. LXX. fig. 2, p, s.0), it is thin, has a 
fenestra on each side, and is crenate along its thin front margin. In front, as is shown 
in the section (Pl. LXIX. figs. 7, 8, s.o), it is pneumatic, and its cavity opens freely 
into that of the right and left epiotic, and at the two fenestre (Pl. LXX. fig. 6, s.o) 
the general cavity is shut in by the parietals, only. ‘The epiotics (ep) finish this part 
of the roof; they are hollow shells, with their concavity looking inwards; this swelling 
contains the junction of the anterior and posterior canals. In the inside views (PI. 
LXIX. figs. 7, 8) the general synchondrosial tract between the periotic elements is 
shown ; it is triradiate, and is large and swollen where the three rays meet. 
The union of these two canals is imbedded in the thick inner bone close to the 
cementing cartilage; but there is a thin table of bone above, and then a large pneu- 
matic cavity between it and the part which contains its share of the labyrinth. In the 
upper view of the epiotico-superoccipital bone (Pl. LXX. fig. 6) a bristle is shown as 
traversing this large upper pneumatic cavity, which is open under the parietal, and 
shows itself again, above, antero-externally, as indicated by the bristle, which is figured 
as emerging from the prootic margin. 
The prootic (Pl. LXIX. figs. 7, 8, pr.o), or foremost of the auditory bony centres, is 
nearly the size of the other two combined, and is not ankylosed to any neighbour bone. 
