TRTASSIC FISHES FROM SPITZBERGEN 
153 
greater powers of resistance, remain, this gives the surface of the bones the granular 
appearance we have mentioned. I was able to observe cell spaces (lacunae) here and 
there in the lamellae. 
Substitution bones of a similar structure are found in Saurichthyids, in Dictyonosteus 
among the Rhipidistids, and partly in the Coelacanthids as well. Woodward (1915, 
p. 3 g, p. 44) has found a bone substance with a similar structure in Lepidotus to. It 
is thus not improbable that this structure is characteristic both for lower Actinopterygii 
and Crossopterygii. On the other hand the conditions of the fossil Dipnoi (Dipterus) 
in this respect are uncertain. I myself have not had an opportunity to investigate 
an^ cranium of these, and no definite information on this matter is found in the litera¬ 
ture at my disposal. 
At least a basioccipital was present in the occipital region of B. mougeoti. The 
anterior end of this bone is still preserved, as it extended in anterior direction under 
the most posterior part of the labyrinth region {Bo, PI. 20, fig. 6; PI. 22, figs. 1, 3 ). In 
cross section {Bo, text fig. 59) this end of it is almost semicircularly curved with the 
concavity dorsally. In the concavity there probably rested the anterior part of the 
chorda dorsalis, which probably extended through the entire occipital region, from which 
it probably also continued some distance into the caudal part of the labyrinth region. 
In the labyrinth region are found two paired substitution bones, one of which 
(Ausph, text fig's. 61, 64 A, PI. 22, figs. 1, 3 ) is situated in the postorbital process, while 
the other occupies the posterior and middle parts of the region ( Pro.o , text figs. 59, 60, 
61, 62; PI. 20, fig. 6; PI. 22, figs. 1, 3 ; Pl. 24, fig. 2.). 
In a caudal direction the latter bone extends farther than the present Cranial part 
is preserved and it is therefore impossible to decide as to its conditions in its most 
posterior part. In a rostral direction it extends nearly to the exit of the facialis nerve, 
where it is separated by a cartilaginous interspace from another ossification situated in 
the orbitotemporal region (text fig. 62); this cartilaginous interspace forms the most 
anterior part of. the labyrinth region. The height of the bone in question equals the 
whole lateral wall of the neurocranium. From its medial upper edge there issues dorsally 
of the cranial cavity a horizontal longitudinal list ( k, text figs. 59, 60; PI. 24, fig. 2) 
which towards the posterior end of the bone meets the corresponding list of bone of the 
opposite side in the median line, but it is impossible to decide whether a fusion has 
taken place there between them or whether a suture still exists. The dorsal surface of 
this list forms the upper surface of the roof of the primordial neurocranium and is covered 
directly by membrane bones. The ventral surface of the list on the other hand, has, 
as far as can be seen, been joined to cartilage, and this has also been the case with 
the rest of the medial surface of the whole bone in question. Consequently, as I have 
tried to show in text figs. 59 and 60, the cranial cavity was in this region mostly 
surrounded by cartilage. 
To judge from its extension, the large bone just described corresponds not only 
to the opisthotic but also the epiotic and prootic. Below I shall give a more detailed 
account of its homologues. I shall only add here that in the following, for the sake of 
brevity, I shall call it the prootico-opisthotic, and that among the recent forms it seems 
most to resemble Traquair’s, opisthotic -j- epiotic in Polypterus (Traquair, 1871, p. 168). 
Stensio, Triassic Fishes from Spitzbergen. 20 
