OF THE SKULL IN LEPEDOSTEUS OSSEUS. 
475 
turn passes into the investing mass (iv. ) below. There the cartilage is a thick wedge, 
right and left, and the space between the two wedges is filled in by the parasphenoid 
( pa.s .); in front of the apex of the notochord. Where these basal plates pass into 
the capsules there the facial nerve (VII.) is severed, and further outwards the capsule 
has a groove under it in which the fore part of the head of the hyomandibular (Jim.) 
is seen ; it is partly ossified above and also below ms the symplectic. A neat lip, the 
rudimentary “ tegmen tympani,” is seen outside the rod, and above it the ampulla of 
the anterior canal ( ct.s.c .), whose arch also is severed higher up. The bony tracts 
below are parts of the large, infolded, angular interopercular ( i.op ., see also Plate 37, 
fig. 4, i.op.). 
The second, third, and fourth hypo-branchials and the middle of the basi-branchial 
(li.br 2 A, b.br.) are severed, and the gills are seen depending over the fore part of the 
heart (h.). 
Section 19.—The roof is now in the superoccipital region (Plate 36, fig. 4, t.cr.); it is 
twice as thick and only two-thirds the width at this part, as compared with the last. 
Here the hind brain (C 3 .) has only membranous sheets interposed between it and the 
auditory labyrinth, which is deficient in its cartilaginous wall in this, the region of the 
“ meatus internus,” where the auditory nerves (VIII.) enter. The section of the 
hyomandibular (Jim.) is here behind the symplectie ; above its head the horizontal 
canal (h.s.c.) is cut through and the end of the arch of the anterior canal (a.s.c.) is 
seen to come close to the great inner “fenestra” of the capsule. Here, as in Sharks 
and Skates, the basal plate (iv.) projects beyond the capsules, and in the angular space 
the 7th nerve (VII.) is cut through. 
Below, this section is still in front of the notochord, but the parachordals (iv.) bend 
down very near to each other; they are supported by the parasphenoid (pa.s.). Here 
the branchial arches (h.br., b.br.) are cut through near their hinder part; below, their 
gills (g.p.) are seen hanging over the heart (h.), and the interopercular is seen outside 
the hyomandibular (Jim.). 
Section 20.—In this slice (Plate 36, fig. 5) the apex of the notochord (nc.) is cut 
through, and here the auditory capsules have recovered their inner wall; the end of 
the horizontal (h.s.c.) and the part leading to the common sinus of the anterior (a.s.c.) 
and posterior canals are cut across. The vestibule ( vb .) is lessening here, where the 
back of the “ sacculus ” is shown. The hyomandibular is severed behind its head, and 
outside it the interopercular and subopercular are shown in section in the fold (op.). 
Here a ganglionic mass belonging to the 9th and 10th nerves (X.) is brought into 
view; and above, the parietals and one of the temporal series (s.t.) are severed. 
Section 21.—The back of the auditory capsule, with the ampulla and hind part of 
the posterior canal (Plate 36, fig. 6, p.s.c.), are now severed; here the double passage 
for the 9th and 10th nerves and the nerves themselves (IX., X.) are seen; the bony 
laminae are severed that are finding their way into the back of the auditory capsule ; 
in the cavity of the ampulla, and from the foramen over the top of the oblique lower 
