TRIASSIC FISHES FROM SPITZBERGEN 
179 
and I now imagine that the above-mentioned process pd in B. mougeoti indicates that 
a ligament homologous with the alisphenoid pedicle in Amia was present and that an 
-invading growth* of bone had taken place from the parasphenoid in the ventral end 
of the ligament. But I have not found any distinct traces of a fastening for such a 
ligament on the pars alisph'enoidea of the sphenoid. 
As has been pointed out above, it is .rather probable that the parasphenoid in 
B. mougeoti did not form part of the direct boundary of the postorbital cavity or did 
so possibly only farthest forwards. As I mentioned, a cartilaginous lamella probably 
extended from the cartilage in the anterior part of the labyrinth region some distance 
in a rostral direction along the medial surface of the parasphenoid both on the lateral 
and the ventral sides of the cavity. Consequently, if this were correct, the cavity would 
have been to a considerable extent a space in the cranial wall itself or, to put it more 
briefly, an intramural space (Allis). The medial wall of the trigemino-facialis part of 
the cavity would then correspond to the ossified wall that, at least in several Teleosts, 
(cf. Allis, 1909a, pp. 45— 47, 207—208; 1914a, pp. 243, 246—248; 1918a, pp. 244—246) 
separates the real trigemino-facialis chamber (or trigeminus chamber) from the trige¬ 
mino-facialis recess. This recess would apparently have been present in B. mougeoti 
partly bounded by the large trigeminus incisur (V) in the posterior border of the sphe¬ 
noid, partly by the cartilage following behind this bone and belonging to the labyrinth 
region. In Amia and probably in Lepidosteus as well, according to Allis (1909a, loc. cit.; 
1914a, loc. cit.; 1918a, loc. cit.) the trigemino-facialis recess has probably fused with the 
trigemino-facialis chamber, and the wall between them has been resorbed. With this 
assumption the trigemino-facialis part of the postorbital cavity of B. mougeoti, leaving 
out of consideration the possible extra-cranial part, would thus be homologous with the 
trigemino-facialis chamber of Teleosts or at least with the trigeminal part of this chamber. 1 ) 
If the trigemino-facialis part of the cavity were in addition separated from the myodomic 
part have by a membranous septum, the agreement with the Teleostei would have been been 
rather great; if, on the other hand, both parts had been in open communication with 
each other, the conditions would have formed an intermediate stage between those in 
Amia and in Teleosts. 
If the postorbital cavity in B. mougeoti has to a great extent been an intramural 
space it is also evident that its myodome, apart from the extra-cranial, anterior part, 
would on either side be homologous with an antero-lateral part of the myodome in 
Amia and the Teleostei. It would presumably represent only a small part of the com¬ 
partment of the myodome in the Teleosts that Allis (1918 a, p. 243) called the dorsal 
one. If the basal part bsph of the sphenoid were to be somewhat reduced and the 
wall formed by the basal parachordal plate (pc) between the myodomes was 
broken through and partly resorbed, we should get a myodome that, at least in 
its essential parts, would be homologous with that in Amia. If the basal part bsph of 
the sphenoid were reduced to a thin septum too and the origin of the musculus rectus 
externus extended into the fenestration (fen. bas. post, text fig. 63 B; PI. 21, figs. 1— 3 ; 
PI. 22, fig. 3 ) that might represent fenestra basicranialis posterior, and if this muscle 
*) As we know, at least in several cases the trigeminus part alone of the trigemino-facialis chamber may¬ 
be developed in many Teleosts (Allis, 1914a, p. 246). 
23* 
