168 MR. D. M. 8. WATSON ON THE 
anterior part of the brain in Pariasaurus differs from the 
sphenethmoid only in that its lateral walls ave produced backwards 
so as to surround the exit of the optic nerve and reach the basi- 
sphenoid. In my opinion such a change, depending in the end 
solely on the degree to which ossification has proceeded, is not an 
important one, and we are thus justified in calling the bone in 
Pariasaurus also a sphenethmoid. 
_ In the large Coal-measure amphibia such as Pteroplax, which 
I hold to be (in the wide sense) ancestral to both the Rachitomous 
and Stereospondylous Stegocephalia and the Cotylosauria, there 
is a great mass of bone, sheathing the front of the brain and 
passing forward as a septum nearly to the front of the head. 
From such a bone the sphenethmoid of Pariasaurus, like that of 
the later Stegocephalia, is easily derived by reduction. If this 
view be true, it will follow that W. K. Parker was essentially 
justified in identifying the pair of bones in the brain-case of 
Urodeles usually called ‘ orbito-sphenoids” with the frog’s 
sphenethmoid, for they also can be readily derived from the 
Pieroplax ethmoidal complex. 
The bone is also interesting from the light it throws on the 
ethmoid of Therapsids. This bone has been carefully and excel- 
Jently described by Prof. and Miss Sollas in Dieynodon and I 
know it well in Hndothiodon, where it has an essentially similar 
structure. In this type it consists of a short, thick median 
septum which rests on the dorsal surface of the “ vomer,” which in 
Anomodonts is certainly composed of a pair of fused prevomers. 
This septum at the top and the back is split into two branches 
which form a covering to the olfactory nerves, which issue-at the 
sides of the bone at about half its length, and are in front 
separated from one another by the septum reaching up to the 
roof of the skull. This bone is even more like the sphenethmoid 
of the frog than is that of Pariasawrus. There can be no doubt 
that it is homologous with the bone I described and called 
ethmoid in the skull of a Gorgonopsid. 
Nor can there be any doubt in my opinion that the ‘ meseth- 
moid” described in Diademodon by Dr. Broom and myself 
veally represents the lower septal part of this bone, for it lies 
on the dorsal surface of the palate, above the vomer in some 
species and altogether in advance of it in the region of the 
palatines in others. 
There can, I think, from a study of some models of a foetal 
skull of Perameles which Mr. R. W. Palmer was good enough 
to show me, be no doubt that this bone is correctly imterpreted 
as the mammalian mesethmoid. 
If this series of comparisons. be justified, we shall have shown 
that the mesethmoid of a mammal, the ethmoid of Anomodonts, 
Gorgonopsids, and Cynognathids, the sphenethmoid of Batrachia, 
and the orbitosphenoids of Urodeles are all allied bones, and 
that they have all been derived from a condition resembling that 
