MR. W. K. PARKER ON THE OSTEOLOGY OF BALANICEPS REX. 287 
larger, which is continued as an open groove forwards and a little upwards into the 
nasal fossa. This groove is on the side of the upper thick part of the pre-sphenoid : 
the foramen itself, although single, answers to the many foramina of the cribriform 
plate of the ethmoid in mammals in function (for it carries the olfactory nerve), but not 
actually, seeing that the ethmoid of birds is facial and not cranial. The recess in 
which the ‘ rhinencephala’ lie is formed by the diverging plates of the connate orbito- 
sphenoids below and by the frontals above ; and although these lobes are the most 
anterior part of the brain, they are pitched up, as it were, against the roof-bones. 
The optic foramina mark the point where the orbito-sphenoid has become single and 
passes downwards and forwards, losing itself in front in the pre-sphenoid, and becoming 
confluent below with the ‘rostrum’ of the basi-sphenoid. 
A fossa above the optic foramen is the upper landmark of the connate orbito- and 
pre-sphenoids—the neurapophyses and centrum of this semi-catacentric sclerotome, so 
curiously modified in relation to the large eye-balls. The centre of the originally 
membranous wall between the orbits is unusually well developed in Balzniceps, being 
generally arrested more or less in the Bird-class; it is, however, in this bird. all 
converted into bone, the diploé of which is scarcely absent from even the central part. 
Although, for convenience-sake, we have separated the orbito-sphenoid from the pre- 
sphenoid in description, they are nevertheless only parts of one large inter-orbital 
ossification in the class of Birds. 
The thick anterior part of the pre-sphenoid is the first to ossify ; this process com- 
mencing in the Chicken about the twelfth day of incubation, and in the Pigeon near the 
time of hatching. The latter bird is able to fly before any of the orbito-sphenoidal 
region is ossified ; and both the Chicken and the Emeu have attained a considerable size 
before the ossific matter reaches the ali-sphenoids. 
In such birds as the Rail and Water-hen, the Grebe and the Cormorant, the interor- 
bital septum is very incomplete. The leg of the inverted Y-shaped process bounding 
the common optic passage in Gallinula chloropus is an exogenous spur growing back- 
wards and downwards from the pre-sphenoid ; but nearly at right angles with this, 
another spur projects forwards and downwards. ‘This latter spur, with the diverging 
alee that coalesce with the ali-sphenoids just above each optic foramen, might easily 
be mistaken for the actual representative of the little Y-shaped interorbital bone of 
such fish as the Perch and the Sea-bream (Pagellus centrodontus). This latter ossicle 
has indeed been a ‘ bone of contention :’ it is the ‘ sphénoide antérieur’ of Cuvier, the 
‘entosphénal’ of Geoffroy, the ‘os innominatum’ of Hallman, and the ‘ ethmoide 
cranien’ of Agassiz; whilst Professor Owen, calling it the ‘ ento-sphenoid,’ considers it 
to be the internal part of the centrum of the pre-sphenoidal vertebra; but its real 
nature was well seen by Professor Goodsir, who considers it to be a feebly developed 
centrum of the post-sphenoid, or perhaps even of the temporal sclerotome. We do not 
agree with Cuvier that it is part of the anterior sphenoid, nor with Professor Owen that 
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