i8o 
ERIK A : SON STENSIO 
continued to extend backwards between the parasphenoid and the basis of the primor¬ 
dial neurocranium, a myodome would arise that was in all respects homologous with 
that of the Teleostei (cf. Gaupp, 1905; pp. 667 — 669; Allis, 1909a, p. 195; 1918a). 
Finally the possible extra-cranial part that would form the most posterior part of 
the postorbital cavity would be situated between the sphenoid on the one side and the 
parasphenoid and the anterior part of its processus ascendens on the other, as is evident 
from the account given by me above. An extra-cranial part of this sort would of 
course be added to the myodome and the trigemino-facialis chamber in Amia, if 
the parasphenoid of this fish were wider and its processus ascendens extended farther 
forward. 
If we now start out instead from the alternative that the postorbital cavity in 
B. muogeoti was preponderatingly an extramural space, the trigemino-facialis part pro¬ 
bably showed about the same conditions as in Polypterus, i. e. it would have corresponded 
to a trigemino-facialis chamber only functionally (cf. Lehn, 1918, pp. 403—404). With 
regard to the myodome it too could, -under similar conditions, be termed a myodome 
chiefly from a functional point of view. This would also be true of its anterior and 
middle parts. The posterior part, on the other hand, seems, as we have found, on account 
of the development of the fossa my to present such conditions that it is difficult not 
to regard it as homologous to at least a small antero-lateral part of the myodome in 
Amia and the Teleosts. 
The view that the postorbital cavity in Birgeria mougeoti is chiefly an extramural 
space does not, however, in my opinion, fit in well with the development of the orbito¬ 
temporal region as a whole. It is true that in Birgeria mougeoti, as we have seen, this 
region has a number of features in common whith the corresponding region in Poly- 
pterids and Crossopterygians, but as a whole it can be unconditionally characterized 
as Actinopterygian-like. In several respects it even appears to be considerably more 
specialized than that of the sturgeons and approaches that of the higher Ganoids and 
Teleosts. It would consequently be rather strange if this spezialisation had not appeared 
with regard to the postorbital cavity as well, so that not only functionally but also in 
reality it corresponded to more or less considerable parts of the myodome and trige¬ 
mino-facialis chamber of the higher Ganoids and Teleosts. 
A rather important support for the correctness of this view would be afforded by 
my interpretation, given above, of the small process pd on the medial side of the sphe¬ 
noid, provided that this interpretation could be shown to be the right one. 
As in any case I consider that I have proved that the progress leading to the 
development of myodome had already definitely begun in B. mougeoti, there is nothing 
surprising in this. For according to what we know of the myodome in primitive 
Teleosts (Allis, 1909 a, pp. 201—202) and Semionotids (Allis, 1909 a, p. 201; Woodward, 
1893 b, p. 564; Frost, igi 3 , p. 221), we must expect it to occur in more primitive forms 
than the higher Ganoids, and in that case most probably already in Palaeoniscids, a 
view, which, according to the observations made on B. mougeoti, seems to be correct. 
But I do not mean to suggest by this that a myodome is found in all Palaeoniscids. 
As I have tried to show above in this work (pp.62, g 3 , 121) a myodome was probably 
developed in Coelocanthids too. 
