MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
699 
sac. On the anterior and posterior walls of each sac there is a small foramen, which I 
imagine may be the equivalents of the orbital and prenasal openings of the orbito- 
nasal canal of Elasmobranchs. The optic foramen (II) is situated at about the middle 
of the inner wall of the orbit ; above it there is a small aperture ( m.o .) for the 
motores oculi, and behind it are two relatively large foramina for the branches of the 
fifth nerve. 
The Facial nerve (fig. 6, Fa.) on leaving the cranial cavity passes obliquely back- 
wards and outwards through the cartilage of the otic capsule, and instead of opening 
directly on its external surface, communicates with a short antero-posterior canal, the 
outer wall of which is formed by an oblique bar of cartilage strengthened 
externally by the basi-temporal wing of the parasphenoid. The comparatively large 
anterior and posterior apertures of this canal are marked YII and VI I' in fig. 3. 
This obliquely disposed cartilaginous bar resembles a similar process (Knorpelspange), 
described by Gegenbatjr as holding a similar relation to the Facial foramen in 
Rliinchobatus, Trygon, and Pristis. In these Elasmobranchs the anterior opening of 
the canal is for the exit of the Ramus palatinus, while the posterior one transmits the 
common stem of the pre-spiracular and post-spiracular branches of the Facial nerve; and 
it seems probable that the canal in Polyodon, so similar in all other respects, subserves 
a like function. Whether the foramen marked s in fig. 6 is for the pre-spiracular 
nerve, or, as is more probable, simply transmits a vessel, I am unable to say. Imme- 
diately over the pointed apex of each basi-temporal ala the slit-like inferior termination 
(x.) of the canal, whose upper extremity opens into the anterior part of the /-shaped 
groove, can be seen. 
Behind the posterior opening of the Facial canal is the small aperture for the exit of 
the Glossopharyngeal nerve, and still more posteriorly, and a little above it, is the 
large oval foramen for the Vagus. The anterior and posterior roots of the spinal 
nerves pass out of the spinal canal through a series of pairs of obliquely-disposed 
foramina, .the one for the anterior root being inferior to, and slightly in advance of 
that for the posterior root. A supra-orbital foramen passes upwards through the 
posterior wall of the orbit, and opens into the adjacent part of the /-shaped groove. 
Fig. 6 represents a vertical and longitudinal section of the cranium and rostrum. 
“ The notochord, extremely large in the spinal column, rapidly diminishes in size as it 
enters the skull, and becoming a mere thread, terminates behind the pituitary fossa.” 
The haemal canal ( h.c .) extends forwards between the diverging arms of the hinder part 
of the parasphenoid, and terminates where the former meet the body of the bone ; 
and the canal (B) which is included between the upper halves of the neurapophyses, 
lying dorsad of the spinal canal and quite distinct from it, opens anteriorly on the hinder 
part of the cranial roof. The cranial cavity is high, but much compressed laterally. 
It extends forwards between the orbits, and is terminated anteriorly by a lamina per- 
pendicularis, on either side of which the Olfactory nerves diverge to the nasal capsules. 
The spinal canal is enormously dilated in front, and with that part of the cranial 
