154 ERIK A : SON STENSIO 
If we now examine this peculiar prootico-opisthotic of B. mougeoti in detail we 
find the following. The cranium P. 165 is certainly split both along and across and can 
therefore be investigated in several different sections, but in spite of this there is not 
much to observe in the labyrinth region,, as part of this is much affected by pressure, 
so that the cavity for the labyrinth and certain foramina can no longer be clearly 
distinguished. To this is to be added the fact, which has already been emphasized, that 
it consisted partly of cartilage, which is of course not preserved. 
Farthest caudally the prootic-opisthotic (Pro. o, text fig. 59) is rather thin and 
was situated with the ventral half of its medial surface apparently against a strong 
ascending lamella (pi) of the parasphenoid. In the present state of preservation of the 
cranium, it is true, the prootico-opisthotic does not show the same conditions, but is 
somewhat removed from the parasphenoid lamella, pi. Everything goes to indicate, 
however, that this is a secondary condition, due to the com¬ 
pression of the cranium. The parasphenoid lamella, pi, which is 
paired, is about half as high as the neurocranium. Dorsally and 
partly also medially of it cartilage was present around the 
cranial cavity and partly round the chorda as well. Under 
these circumstances it is impossible to obtain an exact idea 
of the extent of this cartilage and the size and shape of the 
cranial cavity. Text fig. 59 is only intended to give an approxi¬ 
mate representation of the conditions with regard to this. 
A transverse section somewhat farther forward than the 
one described is shown in text fig. 60. Contrary to w r hat was 
the case in the preceding section, the medial list of each prootico- 
opisthotic (k) does not reach the median line and becomes 
rapidly thinner medially. The lateral surface of the prootico- 
opisthotic of either side is covered in a dorsal part by a ventral 
longitudinal lamella (vl) from the supratemporo-intertemporal 
(«squamosal»). Anteriorly each prootico-opisthotic has joined the 
cartilage in the anterior part of the labyrinth region. On the 
medial surface of this bone cartilage has certainly been present, and the labyrinth recess 
{lb, text fig. 60) was partly contained in this cartilage but, as has already been said, 
nothing certain can be etablished as to the shape of this recess or its connection 
with the cranial cavity. The parasphenoid lamella pi is strongly developed, although 
it is here of another shape and not quite so high as in the cross-section further back just 
described. It is impossible to decide whether the space between it and the ventral part 
of the prootico-opisthotic was filled with cartilage or bone. As a matter of fact it is 
not improbable that the labyrinth recess extended a little way down between them. 
On the right side of the cranium we find in the most posterior part of the prootico- 
opisthotic a partly exposed canal, running dorso ventrally, which cannot have contained 
anything but the canalis semicircularis posterior {c. sem. post. PI. 22, figs. 1, 3 ). The 
canal is situated with its ventral half laterally of the parasphenoid lamella, pi, which I 
shall call in the subsequent description the profound lamella. I have also been able to 
verify, beyond all doubt, that the can. semicircularis externus (c. sem. ext, PI. 22, fig. 3 ) 
Cross-section through the pos¬ 
terior end of the labyrinth region. 
After P. i65. 3 / 4 . Cartilaginous 
parts dotted. 
Bo, basioccipital; Pro. o, prooti¬ 
co-opisthotic; Psph, parasphe¬ 
noid; ch, chorda canal; cv, 
cranial cavity; k, medial list 
from the prootico-opisthotic in 
the roof ofthe primordial neuro- 
crauium; pi, profound lamella 
of the parasphenoid; si, super¬ 
ficial lamella ofthe parasphenoid. 
