MYODOME AND TRIGEMINO-FACIALIS CHAMBER 269 
(fenestra basicranialis posterior, Gaupp), leaving the carotid 
arteries behind it, in persisting remnants of the fenestra hypoph- 
yseos which I have called, in Amia, the internal carotid canals. 
The carotid arteries do not, in either Amia or the Teleostei, 
enter any part of the dorsal myodomic cavity. In certain other 
fishes and in higher vertebrates they become included in that 
cavity. The arteries must accordingly there have either shifted 
posteriorly, with the hypophysis, out of the fenestra intertra- 
becularis into the fenestra interparachordalis, or the inner walls 
of the canals traversed by them in Amia, both the carotid canals 
through the presphenoid bolster and those parts of the para- 
basal canals which lodge those arteries, must have been re- 
sorbed, the canals thus being added to the dorsal myodomic 
cavity The arteries would then lie dorsal to the cartilaginous 
floor of the myodomic cavity, instead of, as in Amia, ventral 
to it; their foramina would lie near the hind end of the subpitui- 
tary portion of the pituitary fossa, instead of anterior to it; 
and a part of the ventral compartment of the teleostean myo- 
dome would be added to the definitive myodomie cavity. 
The septum interorbitale may now be considered, for it forms 
a direct anterior prolongation of the median vertical myodomiec 
membrane and hence must be of similar origin. This septum 
is said by Gaupp (05 b, p. 585) to characterize the tropibasie 
cranium, and to be found in many of the Selachii (Plagiostomi ?), 
in the Ganoidei, the Teleostei, excepting the Siluridae and 
Homaloptera, and the Amniota. The platybasic cranium, in 
which this septum is wanting, is said to be found in many 
of the Selachii and in all of the Amphibia. In the Teleostei 
the septum is said to lie above the trabeculae (Gaupp, ’05 b, 
pp. 667 and 762), between them and the cavum cerebrale cranii. 
The septum must then be formed by the ventral portions of the 
side walls of the primordial cranium pressed together in the 
median line, and this is in accord with Gaupp’s conclusion in 
his work on Lacerta, where he says (’00, p. 553) that this septum 
must either be a wholly new formation of the tropibasic (tro- 
pidobasic) cranium or be formed from material derived from 
the side walls and floor of the platybasic (homalobasic) cranium, 
JOURNAL OF MORPHOLOGY, VOL. 32, NO. 2 
