260 
MR. W. K. PARKER OK THE STRUCTURE AND 
The splint bones that support the basis cranii, lying on a higher level, are the 
vomer and the “parasphenoidthe former (fig. 6, v.) is here seen to be the common 
keystone of this very perfect double archway of the nasopalatine passages (n.p.c.). 
The keel of the vomer wedges itself in between the ribbed edges and the two palatines, 
and these complete the divisions of the common nasopalatine channel; its broad sides 
then unite by harmony with the orbital plates of the palatines, so that it is conjoined 
with those bones both in the middle and also right and left. These lateral flanges of 
the vomer end as free spikes under the presphenoid ( p.s.), and the middle, shorter 
part is also split. But the other bone just mentioned—the “ parasphenoid ”—is not 
seen in the embryo of G. volans (fig. 1), nor, indeed, in any other Mammal examined 
by me, as yet. 4 ' 
This small style of bone lies under the foremost two-thirds of the basisphenoid ( h.s .), 
but is very distinct from it; it resembles, at first sight, the ridge often seen under 
the basioccipital, which is due to the ossification of the sheath of the notochord ; but 
this tract is wholly pro- chordal. It is the exact counterpart of the parasphenoid of 
the Lacertilia, which disappears as mysteriously in some kinds as in G. volans; in 
the case of two closely related Cyclodonts— Cyclodus nigroluteus and Trachydosaurus 
rugosus: in the latter it has been taken aw T ay, and in the former it has been left. 
Here the Marsupial condition of the well-developed jugal (j.) is seen again (fig. 6,j.), 
and a still more extensive development of the equally Marsupial squamosal ( sq .) with 
its thick, crescentic, hollow postglenoid process, its large pretemporal part clamping 
the alisplrenoid ( al.s .) and its extensive swollen post-temporal plate. The tympanic 
( a.ty .) is, here, most instructive as to the interpretation of the thoroughly ankylosed 
adult skull (Plate 39, figs. 3-8). This bone, now well-formed, is a remarkable shallow 
dish with a most irregular outline, and a crescentic convexity in its deepest part, 
the convex outline of which runs close to the fissure for the exit of the glossopharyngeal 
and vagus nerves (IX., X.). 
Where, as a rough wedge, it runs forwards and reaches the mesopterygoid, there it 
lias a considerable oblique foramen, the Eustachian opening ( eu .); this is surrounded 
by cartilage in the embryo (fig. 1). Externally it is jammed in between the post¬ 
glenoid and post-temporal elevation of the squamosal (sq., see also Plate 38, fig. 4, 
a.ty.). Thus, since birth, it has finished the bowl of the dish and formed the deep 
two-lipped spout—the bony meatus externus. 
In the end view (Plate 37, fig. 7) the parietals (p.) are seen with their lower 
edge forming the top of the temporal fossa, which half is scooped out of the squamosal 
(sq.). The swelling of the post-temporal part of the latter bone (see also Plate 38, 
* I stall have to describe a pair of bones found in membrane, and added to the basisphenoid, when 
I come to other and higher kinds of Eutheria; these have long been known in Man as the “ lingulae 
sphenoidales.” I strongly suspect that they are the “ basitemporals ”—symmetrical remnants of the 
parasphenoid—so well seen in Birds and the Crocodilia (Phil. Trans., 1869, Plate 82, fig. 2, b.t.; and 
Trans. Zool. Soc., vol. 11, plate 66, fig. 3, b.t). 
