14 ON THE STRUCTURE OF EUSTHENOPTERON 
and narrow median bone expanded at its hinder end and above in 
front, which I assume to represent, at least in part, the parasphen- 
oid; although it differs greatly from that in any other known fish. 
This element arises below the junction of the parietals and front- 
als and extends forward as a vertical double sheet of very thin 
bone which in life enclosed either a cartilaginous or a cancellous 
bony interior. This forms a complete interorbital septum and 
extends from the roof of the cranium to the mouth. Its lower 
margin is studded with a shagreen of fine denticles. 
This element is much expanded posteriorly and is excavated 

Text fig. 5. Eusthenopteron foordi, Whiteaves. a, Side view of brain 
case and interorbital septum, including vomerine and _ parasphenoidal 
regions; with vertical cross-sections at various points. The paired olfac- 
tory canals shown in the anterior section have united in that following. 
b, Brain case viewed from behind. 
into two chambers (Text fig. 5) opening backward, the lower one 
conical and pierced on each side by a foramen which opens extern- 
ally into a recess just in advance of a lateral process; the upper 
chamber is trough-shaped, open above as well as at the rear and 
like the lower a foramen leads out of each side. There is another 
foramen a short distance behind the nasal chamber. Above the 
prevomers this bone suddenly expands and is excavated for the 
olfactory chambers which communicate with the surface through 
openings in the premaxille. One specimen which exhibits a 
