608 
ME. W. K. PAEKEE ON THE STEUCTUEE AND 
borrowed bony margin (in front). All that part of the prootic which encloses, and lies 
in front of, the trigeminal nerve is a bony trespass on the posterior sphenoidal region, 
whose bounds are from the optic passage forwards to the hinder surface of the 
trigeminal. 
As in the Ichthyopsida generally, the posterior sphenoid seeks to become (in the 
chondrocranium) a perfect girdle — a “ sclerotome.” Here, as in the anterior sphenoid, 
the trabeculae have each a mural upgrowth ; but in this part the wall-plate thickens, 
and a ceiling is thrown over, which, however, is narrow near the side and then wide at 
the middle, where it is fused with the occipito-otic ceiling. Such a conformation leaves 
skylights in the ceiling of an oval form ; these are the posterior fonianelles , a little 
behind the square, main fontanelle ( lfo.,fo .) ; and both these are filled up with a parch- 
ment of strong fibrous tissue, and roofed with a large shingle of fibrous bone —the fronto- 
parietal (f-P-)- 
But the wall and its wall-plate in the posterior sphenoidal region is a solid concrete 
of endosteal bone, thinly enamelled by ectostosis. The ceiling, above, is mostly unossi- 
fied up to the edge of the foramen magnum ; but, below, the prootic bones, laterally, 
have only a narrow tract of cartilage separating them from the exoccipitals, which bones, 
without a basioccipital threshold or a superoccipital keystone, are all that appertain to 
the occipital girdle-bone. Above, the prootics are most, and the exocciptals least; 
below, the contrary takes place (figs. 8 & 4 ,pro., e.o.). Thus, below, a cross of cartilage 
is seen dividing the bones before from the bones behind ; and this soft part is more than 
floored by the cross-shaped parasphenoid ( pa.s .). The ear-sacs, which in the larva 
(Plate 55, au .) were little cartilaginous egg-like bodies, are now, through the development 
of the great tubular arches above and the sacs below, distended into a new shape from 
within. And to this modification by the swelling and rising within of the ear-labyrinth 
is superadded the coalescence of the auditory mass itself on its inner edges with the 
skull, and the fusion with it on the outside of the facial bars. 
Nothing but a careful analysis of these outstretching wings, and a knowledge of the 
history of their growth, could give any adequate idea of their meaning. Their structure 
will be described now, and their development next (see Plate 55). 
Above (fig. 3, pro.), we saw that the prootic had hardened the wall-plate in the 
posterior sphenoidal region ; behind that part it forms a roof to most of the anterior semi- 
circular canal ( a.s.c .). The bony growth then fails on the inner side, and at the junction 
of that canal with the posterior (p.s.c .) there is only cartilage unusually calcified , an 
initial “ epiotic ” bony centre ( ep .). This rounded, arched elevation has its convex edge 
limited by the exoccipital (e.o.), and its concave or outer edge by the prootic. The prootic 
then dips, where it lies over the interspaces of the semicircular canals, but is convex again 
where it enroofs the horizontal canal ( h.s.c .) ; it then forms a sudden, sinuous margin, 
leaving unossified a large ear-shaped mass of cartilage at the outer edge of the occipito- 
otic wings : this free, soft part is the “ tegmen tympani.” 
This latter part is thick at its edge, runs forwards, then passes inwards, and slopes 
