240 
ERIK A : SON STENSIO 
part of it, situated behind the fossa hypophyseos, has even extended into the labyrinth 
region for a short distance, i. e. it belongs to the chordal region of the cranium. In 
height it reaches from the basis cranii quite or almost up to the cranial roof. It has 
evidently alone the same extension as, in usual cases, the orbitosphenoid (or orbito- 
sphenoids if they are paired), the alisphenoids, the basisphenoid and an anterior part of 
the prootic of the Teleosts together, and thus in this respects bears an striking resem¬ 
blance to the similarly termed bone in Polypterus and Dictyonosteus (cf. p. 53—61, 120—140 
above), but is on the whole considerably more specialized than this, at least in Birgeria, 
in which the conditions are as follows. 
The part situated between the orbits grows somewhat thin basally of the cranial 
cavity, even causing at one place a fenestration, which, as is pointed out above, partly 
corresponds to the fenestra optica of Teleosts. Just postero-dorsally of this fenestration 
is situated the foramen opticum and a short distance behind the latter the foramen 
oculomotorii. The nervus trochlearis has had its exit straight above the fenestration but 
so high up that its foramen is found about half-way between the basis cranii and the 
cranial roof. The exit of the trigeminal roots is marked by a large incisure in the 
posterior margin of the sphenoid (in Acrorhabdus it is presumably quite enclosed in the 
sphenoid) and from the anterior wall of this incisure a canal probably for n. ophthal¬ 
micus profundus passes off into the lateral wall of the sphenoid for a short distance and 
has its exit about half-way between the trigeminus incisure and for. oculomotorius. Just 
in front of and somewhat dorsally of the trigeminus incisure on the lateral surface of 
the sphenoid there is found the posterior opening of a somewhat coarse canal running 
in an antero-dorsal direction. This canal is apparently pierced at least by n. ophthal¬ 
micus lateralis, which probably had its exit from the cranial cavity just above the 
trigeminus incisure. Either of the two carotides internae penetrated upwards into its 
special canal, opening into the cranial cavity just behind and somewhat medially of the 
foramen opticum on its side- A canalis transversus homologous with that of the Elasmo- 
branchii is found strongly developed and situated in the postero-ventral part of the 
sphenoid, somewhat posterior to the place where this is pierced by the canals for the 
carotides interna. The aperture of the transversal canal has on each side clearly begun 
to be affected by the musculus rectus externus, so that we have here a myodome that 
is as yet in so primitive a stage that it is paired. In the roof of the myodome of each 
side n. abducens probably had its exit. 
The distance between the foramen opticum and the canalis transversus is very 
much shortened, a condition that has also strongly affected the fossa hypophyseos, so 
that this has formed a narrow vertical pit in the floor of the sphenoid. Ventrally the 
fossa hypophyseos opens into the canalis transversus, which indicates that the hypo¬ 
physis extended into the transversal canal, and that probably the saccus vasculosus may 
also have been situated there. On the part between the foramen opticum and the trans¬ 
versal canal there issues from the basal edge of the sphenoid towards each side a thin 
and rather large process, probably representing a basipterygoid process. Behind the 
transversal canal the bottom of the sphenoid consisted only of rather thin bone-plate, 
the basal parachordal plate, with a median oval foramen, possibly representing the 
remains of the fenestra basicranialis posterior. 
