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ERIK A : SON STENSIO 
which is weak, may also sometimes be quite absent. The hypophysis is situated in a 
foramen that is bounded anteriorly by the basisphenoid, if this bone is present, posteriorly 
and laterally by a medial process from the prootic of each side. 
When the orbitosphenoid is absent or is reduced, the side walls and the floor of 
the cranial cavity consist farthest anteriorly of membranous tissue connected ventrally 
with the membrane that occupies the fenestra optica and farther ventro-caudally some¬ 
times also with the membrane that forms the floor of the fossa hypophyseos and in addition 
represents to some extent the roof of the myodome. Under the same circumstances by 
far the greater part of the interorbital septum below and anterior of the cranial cavity 
may consist either of cartilage or membranous tissue or of a combination of these two. 
In those Teleostei in which the anterior part of the cranial cavity is reduced n. olfac- 
torius has its course free through the orbit, afterwards entering the ethmoidal region 
through the foramen olfactorium advehens on the planum antorbitale (cf. Gaupp 1905, 
pp. 585—586). N. opticus has its exit into the orbit through the posterior part of the 
fenestra optica, and in certain forms n. oculomotorius has also emerged into the orbit 
through this fenestra. It seems however to be more common for this nerve to have 
pierced the antero-lateral part of the medial process of the prootic which forms the 
«prootic bridge* with that of the opposite side, or for it to have passed through the 
suture between the same medial process of the prootic and the basisphenoid. The 
n. trochlearis has its exit through or just in front of the alisphenoid, and the trigeminus 
either through the most anterior part of the prootic or on the boundary between this 
and the alisphenoid, or else so that its main portion passes through the prootic or on 
the boundary between this and the alisphenoid, while an anterior portion (cf. Sagemehl 
iggi, PP- 568— 599) pierces the alisphenoid. 
In the Teleostei the arteriae carotides internae or their equivalents, art. encephalica, 
if they have been fused into one vessel, have their course immediately behind the so- 
called basisphenoid upwards to the cranial cavity. 
From what has been mentioned it ought easily to be seen that in Teleosts the 
prootic has encroached upon certain of the parts of the neurocranium that are occupied 
by the sphenoid in B. mougeoti. The so-called basisphenoid of the Teleosts corresponds 
in B. mougeoti only to the part of the pars basalis of the sphenoid that is situated bet¬ 
ween the canals for the arteriae carotides internae and the fenestra optica. On the 
other hand it is evident that in B. mougeoti the fenestra optica forms only the ventral 
part of the fenestration with the same name in the Teleosts. 
Amia (Allis, 1897a; Sagemehl, 1884) is, as we know, related to the Teleostei in 
several respects with regard to the development of the orbitotemporal region, but it 
also shows some features that seem to resemble Birgeria mougeoti. Thus in Amia the 
prootic does not extend so far forward as in Teleosts, but the most anterior part of 
the labyrinth region consists of cartilage. Finally it is also noteworthy that r. maxillaris 
and r. mandibularis trigemini together with n. buccalis lateralis have left the trigemino- 
facialis chamber on the boundary between the alisphenoid and the anterior, cartilaginous 
part of the labyrinth region (cf. p. 164 above). 
In Lepidosteus (Allis, 1909a, pp. i 83 —190; Veit, 1907, pp. 188—ig 3 ) the orbito¬ 
temporal region is, as we know, very much lengthened in its anterior part, and the 
