i 62 
ERIK A : SON STENSIO 
description placed it at the line between this ossification and the cartilage following 
behind in the anterior part the labyrinth region. 
As has already been mentioned, the orbito-temporal region is occupied chiefly by 
a powerful bone, which, curiously enough, is unpaired (Sph, text figs. 61, 62, 63 , 64; 
PI. 20, fig. 6; PI. 21, figs. 1— 3 ; PI. 22, figs. 1, 3 ). From the place of exit of the trigeminus 
roots (V) this bone extends forwards to the ethmoidal region or ought at any rate to 
have reached fairly near this region with its anterior end. Apart from its anterior and 
posterior parts its height is equal to or only slightly less than that of the primordial 
neurocranium. In position and extension it thus seems to correspond most closely to a 
sphenoid in the same sense as in Polypterus, or Dictyonosteus (cf. pp. 56—60, 121, 142 above), 
and the account given below will 
show that this view is correct. 
I have accordingly used the term 
sphenoid for this bone. 
For a more exact descrip¬ 
tion it is proper to distinguish 
on the sphenoid of B. mougeoti 
an unpaired pars basalis and two 
partes laterales, one issuing along 
the right and one along the left 
Text fig. 62. Birgeria mougeoti Agassiz. 
A somewhat schematized sagittal section through the lateral wall of the neuro- 
cranium. After P. i65. 3, 4 . Section surfaces through cartilage bones marked 
with a fine reticulum. The restored cartilaginous parts are dotted: 
Pro. o, prootico-opisthotic; Psph, parasphenoid; Sph, sphenoid; at, anterior 
portion of the pars lateralis of the sphenoid; a. opt, foramen for art. optica?; 
b, process on the posterior margin of the pars lateralis of the sphenoid; bsplt, 
portion of the pars basalis of the sphenoid between the fenestra optica and 
canalis transversus; c, process on the posterior margin of the pars lateralis 
of the sphenoid; c. transv, canalis transversus (pituitary canal); d, portion of the 
pars basalis of the sphenoid anterior to the fenestra optica ; fen, opt, fenestra 
optica; my, fossa in which muse, rectus externus had its origin. The fossa 
correspondends to a part of the median section of the myodome in Amia 
and Teleosts; oplith., lat, probable exit of the n. ophthalmicus lateralis; 
pc, basal parachordal plate of the sphenoid ; pi, profound lamella of the para¬ 
sphenoid; v 4 , foramen probably for vena cerebralis anterior; II, foramen 
opticnm; III, foramen oculomotorii; IV, foramen trochlearis; V, place of 
exit for the trigeminal roots; v,, canal for n. ophthalmicus profundus or a 
somewhat equivalent nerv (longitudinal section); VII, assumed position of 
the exit for n. facialis. 
border of the dorsal surface of 
the pars basalis (text figs. 62, 
63 , 64; PL 20, figs. 6; PI. 21, 
figs. 1— 3 ; PI. 22, figs. 1, 3 ). 
The pars basalis (d, bsph, 
pc) extends caudally somewhat 
farther than the partes laterales; 
rostrally, on the other hand, it is 
considerably shorter than these 
(text figs. 62, 63 A, B, C; PI. 20, 
fig. 6; PI. 21, figs. i — 3 ; PL 22, 
figs. 1, 3 ). Its anterior end is 
truncated. Its anterior half (d) is in the anterior portion much more narrow farthest 
ventrally than dorsally and a transverse section through it consequently shows the shape 
of a triangle with the point ventrally and the^ base dorsally. In the posterior portion 
of the same half (d), however, the appearance of the cross-section is somewhat com¬ 
plicated, owing to a fenestration (fen. opt , text figs. 62, 63 , 64 A; PL 20, fig. 6; Pl. 21, 
figs. 1— 3 ) between the two orbits, and this fenestration, as we shall see, corresponds 
to a ventral part of the fenestra optica in the Teleostei and higher ganoids. Dorsally this 
fenestration is separated from the cranial cavity by a thick and fairly wide mass of bone 
(text figs. 63 C, 64 A), ventrally, on the other hand, it is bounded only by a narrow rib 
of bone lying against the dorsal surface of the parasphenoid (Psph). — As will be obvious 
from what has been said above, the ventral surface on the anterior half of the sphenoid 
is reduced nearly to an edge (text figs. 63 B, 64 A; Pl. 21, figs. 1— 3 ; Pl. 22, figs. 1, 3 ). 
