OSTEOLOGY OF THE SKULL OF THE DIMETRODON ro 
exactly the same position, but are relatively much larger than figured 
by Broom. 
Posteriorly the parasphenoid is much reduced, and is attached as a 
slender vertical plate of bone to the anterior end of the basisphenoid 
between the basiptergoid processes. Above, the parasphenoid articu- 
lates directly with a thin vertical plate of bone which expands antero- 
Fic. 6.—Lower jaws of Dimetrodon incisivus (?), showing the inner and outer 
surfaces. Full length of jaws, 33°™. 
posteriorly as it rises in the skull, and finally articulates in the median 
line with the under side of the frontals. This bone can only be 
the ossified ethmoid portion of the median cartilaginous septum. 
The anterior edge of the ethmoid is somewhat irregular and thin, 
and represents the true vomer; the posterior inferior angle is rounded 
and thickened. and there is an excavation which evidently marks the 
exit of the 11 nerve from the skull. The presence of a median septum 
of this character is very peculiar in view of the fact that there are well- 
developed epipterygoid bones indicated by the preserved lower ends 
in contact with the posterior portion of the pterygoids. 
