MYODOME AND TRIGEMINO-FACIALIS CHAMBER 21 
so fused in the basioccipital region also. In the tropibasic cran- 
ium they have been forced apart, doubtless by pressure of the 
eyeballs, and the interorbital septum is formed from the ma- 
terial of the hypochordal bands and the tissues between them 
and the epichordal bands, the latter bands forming the floor 
and side walls of this part of the cranial cavity. The trabec- 
ulae might then be said by certain authors to lie at the ventral 
end of the interorbital septum, and by others to lie at its dorsal 
end. This would also explain how, in fishes where the inter- 
orbital and internasal septa are directly continuous with each 
other, the trabeculae are said by certain authors to form the 
ventral edge of the internasal septum, and by* certain others 
to form its dorsal edge (Allis, 713). 
LEPIDOSTEUS OSSEUS 
In Lepidosteus there is no functional myodome, but the pre- 
existing spaces which correspond to both its dorsal and ven- 
tral compartments occur and were fully described by me in my 
work on the mail-cheeked fishes. The space representing the 
dorsal compartment lies, as does the functional myodome of 
Amia, dorsal to the cartilage which actually forms the basis 
cranil, the space that represents the ventral compartment lying 
ventral to that cartilage, between it and the underlying para- 
sphenoid, and lodging, as in Amia, the internal carotid arteries 
and the palatine branches of the facialis nerves. Veit (’07), in 
a work that did not appear until after my own was sent to press, 
had previously described, in a 150-mm. specimen o’ this fish, 
the space representing the prootic portion of the dorsal com- 
partment, calling it the cavum saccivasculosi, and he later (711), 
described, in younger specimens, the development of the car- 
tilages that bound that space. 
In 8 to 16-mm. embryos of this fish, Veit (’11) says that the 
notochord is the only recognizable skeletal element; and it ends 
with a blunt point against the hind wall of the infundibulum, its 
extreme tip turning slightly ventrally. In embryos 10 to 11 
mm. in length the notochord is in similar position, but three 
cartilaginous elements have now developed on either side of 
