TRIASSIC FISHES FROM SPITZBERGEN 
i63 
The posterior half of the pars basalis of the sphenoid (bsph, pc), which begins im¬ 
mediately behind the fenestra optica, has quite a different shape from that of the anterior 
half. The width is about as great dorsally as ventrally. The transverse section has conse¬ 
quently a somewhat rectangular shape, and we can distinguish a dorsal and a ventral sur¬ 
face besides a right and a left lateral (text figs. 63 , 64 B; PI. 20, fig. 6; PI. 21, figs. 1— 3 ). 
Text fig. 63. Birgeria mougeoti Agassiz. 
The sphenoid. After P. 16S, 3 / 4 . The section surfaces shown by reticulation. 
A) Sphenoid from the right side; B) from the ventral side; C) in a median sagittal section. 
Psplt, parasphenoid; a, a p corners on the edge between the latero-dorsal and the lateral surfaces; a. car. ini, foramen for arteria 
carotis interna; a. opt, foramen presumably for arteria optica; al, anterior portion of the pars lateralis of the sphenoid: b, c, 
processes on the posterior margin of each pars lateralis of the sphenoid; bp, basipterygoid process; bsph, portion of the pars 
basalis of the sphenoid between fenestra optica and canalis transversus; c. transv, canalis transversus (pituitary canal); d, portion 
of the pars basalis of the sphenoid anterior to the fenestra optica; dors, sell, dorsum sellac; fen. bas.post, remains probably of 
fenestra basicranialis posterior; fen. opt, fenestra optica; Ji, incisur probably pierced by n. abducens; /. hyp, fossa hypOphyseos; 
my, fossa in which musculus rectus externus has originated. Corresponds to a part .of the middle part of the myodome in Amia 
and Teleosts; ophth. lat, probable place of exit for n. ophthalmicus lateralis, ophth. lat v posterior opening of the canal for 
ophthalmicus lateralis; ophth. lat 2 , anterior opening of the same canal; pc, the basal parachordal plate of the sphenoid; r, canal 
for vessel and nerve branches to the cranial roof; v p v 2 , v 3 , v s , foramina for vessels and nerve-branches; v 4 , canal probably for 
vena cerebralis anterior; II, for. opticum; III, for. oculomotorii; IV, for. trochlearis; V, incisur indicating the place of exit for the 
trigeminal roots; V ± e, outer opening of the canal for n. ophthalmicus profundus as a somewhat equivalent nerve; V t i, inner opening 
of the same canal. 
In the posterior half of the pars basalis of the sphenoid we can distinguish an 
anterior portion, bsph, comparatively high and massive, and a posterior one, pc, lower 
and more plate-like. On the dorsal side these portions pass into each other without any 
very distinct boundary, whereas this is not the case on the ventral side. For, correspond¬ 
ing to the anterior part of the plate-like region pc, we find on this side a rather deep 
and broad transverse furrow (c. transv, text figs. 62, 63 ; PI. 20, fig. 6; PI. 21, figs, r, 3 ), 
