170 
MR. W. K. PARKER OK THE STRUCTURE ARE 
a new and thick cartilage appears ; this is the symplectic (sy.), it is crescentic in this, 
its front edge (lower end), and between it and the quadrate there is a small nucleus 
of cartilage which will be described in the next stage—it is a “ suspensorial ray ” (sp.r.). 
The chief scutes seen are the parietals; the squamosal, the opercular, and the sub- 
opercular, also, may he traced at this point, hut are not figured. 
Section 16.—In this slice (Plate 17, fig. 8) we see several new parts come into view 
below; above, the skull is altogether flatter, and the supercranial valley narrower. 
The auditory nerve (VIII.) is still shown, also the arch of the anterior and hori¬ 
zontal canals (see also fig. 7, ci.s.c., h.s.c). The basal plate (iv. , nc .) is very irregular, 
as it passes into the auditory capsules, right and left; the parasphenoid ( pa.s .) partakes 
of this irregularity, and here its decurved wings are larger. The compound meta¬ 
pterygoid plate is composed at this hind part of a large middle, two large lateral, and 
two lesser intervening plates (- mt.pgmt.pg".). The top of the hyomandibular is 
just seen and most of its great protractor muscle ( pr.hm .); below, the symplectic 
is cut through from top to bottom. Below the wide oral cavity the cerato-hyal (c.hy.) 
with a thin ectosteal coating, the hypo-hyals (Ji.hy.), the fore part of the basi- 
branchial ( b.br .), and the fore end of the first hypo-branchials (Ji.br 1 .) are also seen. 
The scutes are very similar to those of the last. 
Section 17.—In this (Plate 17, fig. 9), the cranial valley is seen to be wider again ; 
the auditory sacs are cut through where the posterior canal ( p.s.c .) is descending, and 
the whole structure is lessened laterally; a thin pterotic eave (sj).o., read pt.o.) grows 
down on each side. Here the hind-brain (C 3 .) is less, and it is giving off the roots 
of the large vagus nerve (X.) whose ganglion root and stem are laid bare. Here we 
still see that the auditory sacs are open to the skull (or only closed by membrane), 
and here the sacculus (s.) is seen hanging from the rest of the vestibule (vb.). The 
notochord (nc.) is becoming larger, and the thick curved basal plate (iv.) is here 
separate from the auditory capsules; below it the parasphenoid (pa.s.) is thick, 
and at its notched sides we see a pharyngo-branchial (p.br.) and outside this part 
of the gills (g.). The lower part of the hyomandibular, part of the symplectic, inter- 
hyal, and cerato-hyal (Jim., sy., i.hy., c.hy.) are cut across ; and near the mid line the 
basi-branchial, the first cerato- and hypo-branchials, and the second hypo-branchials 
(b.br., c.br\, h.br 1 ., h.br s .). 
Section 18.—In this section (Plate 17, fig. 10) the post-auditory fontanelle is 
just missed, and we see that it must answer to the interspace between the supra- 
occipital and first vertebral arch ; for the chondrocranium (see Plate 15, fig. 13) sends 
its copious growths over the proper spinal region. The cavity here is very large, this 
being, indeed, the “ foramen magnum/' and the ex-occipital walls are very thick, 
and almost vertical. The hinder part of the vagus nerve (X.) is still seen in the 
chink wdiich separates—up to this point the sides form the base, which is increasingly 
massive, but has not a very large notochordal section (nc.) in it. The parasphenoid 
(pa.s.) is beginning to break up at the middle; outside it a pharnygo-branchial is 
