TRIASSIC FISHES FROM SPITZBERGEN 
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interorbital wall forms there only a thin septum. A myodome is lacking, while the 
fenestra optica is developed. Only two of the substitution bones of the primordial 
neurocranium fall exclusively within the orbitotemporal region, namely the orbitosphenoid 
and the alisphenoid. The anterior part of the prootic, however, as in the Teleostei, also 
extends into the orbitotemporal region and joins the alisphenoid by means of a suture. 
The ventero-lateral part of the prootic forms the greatest part of the basipterygoid 
process of each side. Allis (1909 a, p. 189) has described the conditions immediately 
round the fossa hypophyseos as follows: «Immediately anterior to the cross-canal there 
is, in the floor of the cranial cavity, the pit like and slightly oval pituitary fossa. The 
lateral and anterior walls of this fossa are of cartilage, the floor of the fossa is per¬ 
forated by a nearly circular opening, the opening being closed ventrally by the para- 
sphenoid.» 
The foramen trochlearis is situated in Lepidosteus in the alisphenoid, the foramen 
trigemini in the anterior part of the prootic, the foramen oculomotorii in the suture 
between this bone and the alisphenoid, while the foramen for the profundus branch is 
situated immediately behind the foramen oculomotorii, also in the suture between the 
prootic and the alisphenoid. N. olfactorius has a free course for a long distance through 
the orbit. The carotis interna enters the cranial cavity through the cartilaginous floor 
in front of the fossa hypophyseos. The vena pituitaria traverses the fossa hypophyseos 
in a transversal direction, and in order to emerge into the orbit of each side pierces 
the cartilaginous part of the cranial wall that lies between the posterior boundary of 
the fenestra optica, the prootic and the alisphenoid. 
The part of the floor of the orbitotemporal region in Lepidosteus that corresponds 
to the region bsph in B. mougeoti is, as can easily be seen from the facts given above, 
preponderatingly cartilaginous. It is also noteworthy that it is fairly weakly developed. 
V. pituitaria seems in Lepidosteus to occupy pretty nearly the same position as in 
B . mougeoti , and I cannot see any special reason to assume that it should not occupy 
its normal position (cf. Allis, 1914 a, p. 242). The part of the basipterygoid process in 
Lepidosteus that is formed by the prootic seems to be homologous at least with the 
posterior part of the process bp in B. mougeoti. 
In Polypterus (Traquair, 1871, pp. 169— 1 71; Lehn, 1818, p. 384) an unpaired sphe¬ 
noid extends, as we know, through the whole orbitotemporal region and forms, as has 
been pointed out above, the roof for the cavum sacci vasculosi. The sphenoid is pierced 
by the nervus opticus, n. oculomotorius, n. trochlearis, r, profundus trigemini, arteria 
carotis interna 1 ) and a number of veins, inter alia v. pituitaria. The roots of trigeminus 
have their exit from the cranial cavity trough a foramen situated on the boundary 
between the sphenoid and the anterior cartilaginous part of the labyrinth region. The 
lateralis fibres, with the exception of those destined to the preoperculo-mandibular sensory 
canal, all emerge behind the sphenoid through a special foramen quite surrounded by 
cartilage. (Lehn, 1918, pp. 394—895). N. abducens accompanies the trigeminus roots. 
The sphenoid in Polypterus thus shows in many respects a striking agreement with 
the similarly termed bone in Birgeria mougeoti . In spite of this, however, it is evident 
x ) Arteria carotis interna in Polypterus enters the cranial cavity on each side through or close, behind the 
foramen opticum, thus far anterior of the cavum sacci vasculosi. 
