Cu. I.] OF THE DODO. 87 
‘its respective fossa; at their exit from the apex of the cerebral cavity, they diverge slightly from each 
other, and are separated by an interval of about four lines, corresponding to the breadth of the interorbital 
septum. 
Before describmg more minutely the formation of the olfactory fossee in the Dodo, it will be necessary 
to consider them first in other Pigeons, by which we shall alone gain a correct conception of several peculi- 
arities in the cranium of this extinct form. In all Pigeons, as in many other birds, the anterior extremity 
of the vertical osseous plate, forming the interorbital septum, advances beyond the junction of the nasal 
with the frontal bones ; and is completely covered by the former, which meet in the median line posteriorly, 
but are separated anteriorly by the extremity of the nasal process of the premaxillary; hence no part of it 
appears mesially, behind the premaxillary and between the nasals, as in the Hmew and other Struthionide. From 
each side of the expanded upper border of this advanced portion of the septum, a thin lamina passes hori- 
zontally outwards ; contracting rapidly from before in the antero-posterior diameter, it bends downwards 
and inwards, arching over the foramen for the transmission of the olfactory and ophthalmic nerves and ac- 
companying vessels, to meet and be continued for a greater or less extent along the outer border of a 
vertically transverse, subtriangular plate, projecting outwards from the interorbital septum: this last com- 
mences beneath the common aperture, and increases in breadth as it descends; by its anchylosis with the 
inferior extremity of the prefrontal, it forms the anterior wall of the orbit, separating it from the open 
olfactory cavity in front. 
. 
For reasons which cannot be discussed here, I regard the interorbital septum as the 
compressed body of the third and last, or most anterior of the cranial vertebra ; and the 
processes just mentioned, as ossified portions of the ethmoid or olfactory capsule ; the superior 
I have hitherto denominated the turbimated, and the lower, the inferior ala of the ethmoid, 
and I shall continue to use these terms in the remainder of this description. By the 
sphenoidal rostrum, or rostrum simply, I understand the anterior prolongation of the sphenoid 
which supports the interorbital septum ; it has been incorrectly considered as homologous with 
the anterior sphenoid in mammals, and hence has received the special appellation of presphe- 
noid in Professor Owen’s late paper on the Vertebrate skeleton ;’ whereas the interorbital 
septum in birds is the homologue of the mammalian presphenoid. The bone which has 
heretofore been denominated the lachrymal in birds, is undoubtedly the homologue of the 
prefrontal in the cranium of fishes and reptiles ; the true lachrymal bone, which is external to 
the lachrymal duct, exists in certain Saurians, and in the Crocodilide ; it does not occur in 
the higher Vertebrata, 4ves and Mammalia, while the prefrontal only disappears in certain 
exceptional instances among mammals ; in birds and mammals it has erroneously been re- 
garded as the true lachrymal, and is so named even by the learned Hunterian Professor ; 
this false homology masks one of the most beautiful instances of the unity of organization. 
Having thus explained the meaning of the terms employed, we may return to our 
subject :— 
The fissure remaining between the turbinated ala and the prefrontal, which in many birds transmits 
the upper diverticulum of the suborbital smus, in several Pigeons, is diminished by the extension upwards of 
' Reports of British Association, 1846. 
2A 
