694 
MR. T W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
anteriorly in projections which reach nearly to the antorbital region. The slightly 
contracted orbital region is limited in front by the almost pedunculated nasal capsules. 
Behind the auditory region the cranial roof becomes constricted into a banddike 
area, which is formed partly by the coalesced apices of the anterior neural arches, and 
partly”' by the roof of the occipital region. The divergent processes (</x/>) are the 
posterior ends of the decurrent flaps of cartilage previously mentioned. 
The floor of the occipito-periotic region, where it rests on the parasplienoid, is very 
narrow, but the external walls diverge outwards as they pass upwards to join the 
margins of the flat and comparatively broad upper surface of this part of the skull ; 
hence a transverse section is almost triangular in shape. The width of the cranium 
in this part is mainly due to the massive periotic sacs, and the growth of cartilage 
round the semicircular canals is thick enough to prevent them affording any external 
indications of their position. The upper and external margin of each otic capsule is 
produced outwards into a prominent pterotic ridge, and forwards into a stout sphenotic 
process, which arches outwards, downwards, and forwards, and also forms a roof to the 
hinder part of the orbit. Posteriorly the pterotic ridges terminate in the rounded 
blunt projections which form the postero-external angles of the otic capsules. 
The outer face of each capsule is traversed by a deep vertical groove with a pro- 
minent anterior lip, which extends from the pterotic ridge to a point just above the 
facial foramen, and serves for the articulation of the hyomandibular. This groove, 
limited in point by its projecting anterior border, is seen in fig. 5 ( lvy.g .), as an emar- 
gination of the pterotic ridge behind the sphenotic process. The mesial portion of the 
cranial roof forms an elongated band-like area, flattened, like the pterotic ridges, by 
the impress of the overlying membrane bones — the parietals and frontals ; but on each 
side of it, in the roof of the otic capsule, there is any-shaped groove ( fg .), the anterior 
end of which diverges outwards into the sphenotic prominence, while its hinder end 
terminates on the postero-lateral margin of the pterotic ridge. The groove is visible 
externally through the oblong fontanelle, which on each side is left between the post- 
temporal, dermo-sphenotic, and parietal splints (fig. 1). In the posterior paid of 
each groove there is an infundibuliform opening (p.f), which is the outer termi- 
nation of a short but wide canal leading into the cranial cavity ; and in the anterior 
part there is a much smaller foramen, which communicates by a short narrow canal 
with a slit-like aperture (figs. 3 and 4, x.) on the infero-lateral surface of the otic 
cartilage, between the metapterygoid ligament and the articular groove for the re- 
ception of the head of the hyomandibular. These foramina will be referred to sub- 
sequently. In front of the wing-like sphenotic prominences the chondrocranium is 
constricted to form the orbits. A slight backward outgrowth from the upper margin 
of the ethmo-trabecular region, together with the produced orbital margins of the 
sphenotic process and pterotic ridge, furnish a partial roof to the orbit. Previous to 
the removal of the dermo-sphenotic bone (c 2 ), which extends continuously over each 
* As represented in Plate 56, fig. 5, tliis band-like area is one-sixteentli of an inch too wide. 
