i?6 
ERIK A : SON STENSIO 
decreases rapidly in height and finally disappears entirely at the posterior end of the 
basal parachordal plate of the sphenoid. Its approximate height and extension through 
the whole labyrinth region are shown in text fig. 62 and PI. 20, fig. 6. 
The parasphenoid in the whole has its dorsal surface concave throughout its length 
and its ventral surface convex from one side to the other. This character is most pro¬ 
minent in the orbito-temporal region, where the ventral parts of the sphenoid are sunk 
as if in a groove (text figs. 64 A, B); Cross-sections through the parasphenoid on the 
one hand and the prootico-opisthotic and cartilaginous parts on the other are shown in 
text figs. 59 and 60. 
The exit for n. glossopharyngeus has been situated so low that even the para¬ 
sphenoid (IX, text fig. 61) helped to form the boundary of the outer opening for 
its canal on the ventral side. 
The processus ascendens of the parasphenoid (pr. asc. text figs. 61, 64 B; PI. 22, 
figs. 1, 3 ) is, as already mentioned, powerful and extends dorsally almost up to the 
cranial roof. Immediately behind this process there is in the lateral border of the para¬ 
sphenoid a characteristic sinus, which indicates the position of the posterior openig for 
the canalis facialis (VII, text fig. 61; PI. 22, figs. 1, 3 ). 
The parasphenoid as a whole consists of a compact radiate bone substance than 
can be split up into thin lamellae; it also has a well developed ossification centre 
situated under the canalis transversus. In the profound lamella it has a very special 
character to which no parallel can be found in recent forms. Among fossil forms there 
are perhaps found indications of a lamella of this sort, as we shall see in Saurichthys. 
Whether this was also the case in other forms is not yet known (cf. Day 1915, pp. 422 
—434; Fritsch 1895, pp. 79, 91, 99, 116; Jaekel 1903; pp. 46—50; Traquair 1877 a, p. i6)i 
Apart from the profound lamella the parasphenoid in B. mougeoti seems to be of 
a similar type to that of the recent Sturgeons. It would thus be rather considerably 
different from the corresponding bone in Palaeoniscids in general, which, as far as we 
know, would not have extended so far backwards as in the recent Ganoids and 
Teleosts and which, in addition, has the part that is situated behind the centre of 
ossification very wide. An additional characteristic, at least for certain Palaeoniscids 
that are so far known, is that the parasphenoid on the dorsal surface has a fossa 
developed at about the place of the centre of ossification, and also that the part behind 
this centre sends out on either side two processes in a latero-dorsal direction. Jaekel 
and Day are of the opinion that the hypophysis was situated in the fossa, and the latter 
investigator even says that because of this the basisphenoid may be' fused with the 
posterior part of the parasphenoid. The anterior of the two processes is accordingly 
interpreted by him (Day) as a basipterygoid process, and he considers that he has found 
it developed with a facet for articulation with the palatoquadrate. 
It ought however to be noted that Day’s view does not seem probable in the 
preceding case. The fossa present on the dorsal surface of the parasphenoid in Palae¬ 
oniscids might, it seems to me, very well have been related to the hypophyse without 
for that reason the basisphenoid forming part of the parasphenoid. As an instance we 
may mention that the fossa hypophyseos both in Polypterus and Lepidosteus was bounded 
ventrally by the parasphenoid and that, at least in young specimens of the former genus, 
