476 
ME. W. K. PARKER OK THE DEVELOPMENT 
part of the occipital arch, there are parts of the spreading exoccipitals (e.o.). The 
opisthotic and epiotic are formed over the posterior canal later (see Plate 38). 
The two halves of the investing mass, below, form a rest for the oblique plates of the 
upper part or arch, and are only slightly tilted upwards outside; they are thick inside 
and flattish on the outside, and have the notochord (no.) between them; this rod is, 
here, ensheathed in bone, and this thick bony sheath has sent a wing, right and left, 
over the basal part of the moieties of the investing mass, which are curling over 
their external edge, as far as to the edges of the underlying parasphenoid (pa.s.) ; 
this ectosteal growth is the basioccipital ( b.o.). The top of two of the branchial 
arches ( p.br .), the parietal (at its end), and a supra-temporal bone are also seen. In 
this section the occipital arch is seen to be two-winged, right and left ; in the last 
stage (Plate 33, figs. 12, 13, iv.) the section, here, was like an hour-glass. 
Section 22.—The roof is now (Plate 36, fig. 7, s.o.) very thick, and here the vagus 
nerve only (X.) is seen with its ganglion; the back wall of the auditory capsules 
nearly meet above; the basal plate (iv.), between the halves of which the osseous 
sheath of the notochord (nc.) is seen, with its right and left basioccipital wings (b.o.), 
is surmounted by the side plates of the arch (e.o.). Behind the horizontal canal the 
combined occipital arch and auditory capsules form large thick shoulders of cartilage; 
whilst, above, the arch has three roundish crests (s.o.). Here the basal plate (iv.) is 
thicker, and as in the la,st, the parasphenoid is corrugated; the wings of the basi¬ 
occipital bone (b.o.) do not yet invest the lower face of the cartilage ; the opercular 
(op.), and a post-temporal scute, are seen in section, as also some parts of the hinder 
branchial arches (hr.), with their pectinate gills and grooving vessels. Here the 
peculiar four-fold nature of the occipital arch is well seen, the oblique sides resting 
on a projecting threshold, through which the notochord, with its bony sheath, runs. 
Section 23.—The last of the sections (Plate 36, figs. 8, 9) figured is through the 
thinner hind edge of the occipital ring, which is lozenge- shaped, and somewhat winged, 
right and left, for the side-walls have the same obliquity as the halves of the archway 
above; here the threshold is not so wide as in the last section; it is narrowing towards 
the end of the projecting basioccipital (see Plate 34, figs. 1-3, b.o.). 
The exoccipital ectosteal plate (e.o.) is seen inside the converging arch, the right 
and left plates nearly meet above, and there is no key-stone piece or supraoccipital 
bone in the rounded median part. Here the notochord (nc.) lies impacted between 
the basal plates and their ascending arch; it is the core of a strong basioccipital bone 
(b.o.), which strongly encloses it, the soft tissue spreading in radiating lobes in the 
thickening bone-substance. Laterally, the bone has spread so as to enclose the halves 
of the investing mass, and runs beyond these parts; it grows as a right and left sharp 
plate. Here the wide corrugated parasphenoid (pa.s.) is in two parts, for it is forked 
behind (see also Plate 34, fig. 2, pa.s.). 
