292 PROF. W. K. PAEKEE ON THE STEUCTUEE AND 



canal for the internal carotid artery (i.c), which reappears in the basisphenoid, and ends 

 in the bottom of the pituitary cup (PI. LXX. fig. 4, i.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 (oj)} 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 

 (PI. LXX. fig. 3, e.o). The inner wall of the exoccipital (PI. LXIX. figs. 7, 8, e.o) 

 is a convex 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 (ix, x). The 

 opisthotic (op) bulges towards its fore margin, and is ankylosed to the exoccipital, now, 

 in front of the chink, and projects inwards as a shell of bone, wiiich is angulate in 

 front. Below (PI. LXIX. fig. 8, op) it is narrower, and forms a complete loop of bone 

 round the fenestra rotunda (f.r) and divides it from the/, ovaiis (fs.o). 



The supraoccipital is entirely on the roof of the skull (Pis. LXIX. and LXX. s.o) ; it 

 also is compound, now, for it has coalesced with the right and left epiotics (ej]). 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 (PI. LXIX. figs. 7, 8, 

 s.o), and also above over its junction with the epiotics (PI. 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 (PI. LXX. fig. 2, p, s.o), 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 (PI. LXIX. figs. 7, 8, s.o), it is pneumatic, and its cavity opens freely 

 into that of the riglit and left epiotic, and at the two fenestne (PI. 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 (PI. 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 (PI. 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. 



