700 
MR. T. W. BRIDGE OR THE OSTEOLOGY OF POLYODOR FOLIUM. 
cavity which lies behind the foramina for the Vagi, forms the comparatively high but 
compressed portion of the cranio-spinal canal. In front of these foramina the cavity 
gradually diminishes in height and width towards its rounded anterior termination 
against the lamina perpendicularis. As in other Ganoids and in all Teleostei, the 
inner wall of the periotic capsule is incomplete, the recess in which the membranous 
vestibular sac is lodged communicating widely with the cranial cavity. The letters 
a.sc., p.sc., and h.sc., mark the openings of the semicircular canals into the vestibular 
recess. In the middle of the outer wall of this recess there is a small fenestra (a) 
which opens into the Facial canal behind the point where the seventh nerve enters 
the latter. The condition of my specimen would not admit of a successful dissection 
of the cranial nerves, hence I am unable to say detinitely what vessels and nerves were 
transmitted by the closely aggregated foramina which perforate the side walls of the 
anterior part of the cranium. 
The numerals VII (fig. 6) indicate the cranial end of the passage by which the 
Facial nerve reaches the Facial canal ; V is the internally single aperture for the exit of 
the Trigeminus, and in front of it the small and shallow pituitary fossa is seen to be 
completely floored by cartilage. The small foramen ((7a.) on the anterior margin of 
the fossa may serve for the transmission of the carotid artery, while the small apertures 
indicated by the letters m.o. and tr. probably transmit the motores oculi, and the 
troclilearis. The external opening of the last- mentioned nerve is just in front of that 
for the fifth nerve ; * II is the optic foramen. All these passages pass very obliquely 
outwards and forwards through the cranial walls. The Olfactory nerve breaks up into 
a number of small filaments, each of which reaches the olfactory mucous membrane 
through a separate opening in the inner wall of the nasal capsule, thus giving rise to a 
true cribriform plate. So far as I am aware, in all other branchiate Ichthyopsida this 
nerve passes bodily through a single large foramen to reach the interior of the nasal sac. 
The cavity of the rostrum is quite distinct from the cranial cavity, and terminates at 
some little distance behind its anterior end, which is much depressed and solid: It com- 
municates with the exterior by the single median fenestra in its roof ( a.m.f ), and by 
two lateral foramina (z.). It seems to me that this singular rostral cavity may be 
compared to the large “ meso-etlnnoidal fat cavity,” which Mr. Parker! has described 
as existing in the adult Salmon, in front of the coalesced orbito-sphenoids and lamina 
perpendicularis. Both occupy similar morphological areas, and each has a superior 
median foramen situated beneath the azygous dermo-etlnnoid splint. If this com- 
parison be correct, and the meso-etlnnoidal fat cavity of Salmo is truly homologous 
with the rostral cavity of Polyodon and the Elasmobranchs, then its origin, as the result 
of a secondary fenestration of the internasal cartilage, is another example of a not unfre- 
quent phenomenon in developmental history — the appearance of particular structure as 
a secondary feature, .though normally and in the lower types it may be a primary one. 
* The aperture v seems to transmit a vessel. 
t Parker, “ On the Structure and Development of the Skull in the Salmon.” Phil, Trans., 1872, p. 108. 
