096 
MR. T. W. BRIDGE ON' THE OSTEOLOGY OF POLYODON FOLIUM. 
great dilatation of the neural canal, which is closed above only by a membranous 
fontanelle.” This description, in so far as it refers to Polyodon , .is not quite accurate. 
There is, it is true, a large posterior median fontanelle in the Sturgeon which extends 
forwards for a short distance in front of the Vagus foramen, and is therefore, in part at 
all events, a cranial fontanelle. But in Polyodon I could find nothing at all com- 
parable to this large and conspicuous fontanelle of Acijpenser. The neurapophyses of 
the anterior .vertebrae coalesce over the spinal cord and anteriorly blend with the car- 
tilage of the occipital roof, and the continuity of the roof of the spinal canal seems to 
be uninterrupted. 
We may now consider the foramina which 1 have already referred to as existing in 
the /’-shaped groove in the roof of the periotic capsula. Each infundibuliform orifice 
is situated on the outer side of the common canal by which the anterior and posterior 
vertical semicircular canals open into the membranous vestibule, and the short but 
relatively wide passage into which the orifice leads passes downwards and inwards 
beneath the arch of the posterior canal, and opens into the cranial cavity just behind 
the recess in which the vestibular sac is lodged (Plates 56 and 57, figs. 5, 6, and 7, pf.) 
These foramina at first appeared to correspond to the paired posterior fontanelles 
which are seen in the roof of the Frog’s chondrocranium when the overlying parieto- 
frontal bones have been removed, or to similar vacuities existing in the roof of the 
Salmon’s skull, which in the latter are spaces left on each side of a recurrent growth 
of cartilage derived from the ethmoidal region. But these fenestrse have nothing to 
do with the otic capsule ; they are always mesially placed with regard to it, and their 
passages of communication with the cranial cavity do not pass beneath the arch of the 
posterior vertical canal, or in any way perforate the periotic cartilage. 
Another alternative is to regard these infundibuliform orifices and their canals as 
being due, like the parietal foramina of the Elasmobranchs, to the persistence of the 
canals by which the primitive auditory involutions of the embryo Polyodon com- 
municated with the exterior. But this view may be met by the objection that in the 
Selachii the parietal foramina and their canals do not pass beneath the arch of the 
posterior vertical canal, but are situated altogether to the mesial side of the vestibule 
and its canals. Moreover, in the Shark these foramina do not communicate with the 
cranial cavity but with the central cavity of the periotic cartilage in which the 
membranous vestibule is lodged. In mitigation of the force of the first objection, it 
may be urged that in the young Polyodon the coecal outgrowth from the vestibule, 
which eventually becomes the posterior vertical semicircular canal, may have grown 
round the primitive involution from the exterior instead of having its growth limited 
to the outer side of that ingrowth, as appears to have been the case in the young 
Shark. In opposition to the second objection it may be said that if the inner wall of 
the periotic capsule be atrophied, as it is in ail Teleostei and Ganoids, then the parietal 
foramina will necessarily communicate with the cranial cavity, as do the infundibuliform 
orifices in Polyodon. 
