The orbito-sphenoids, or lower segments of the fronto-neural arch, are rarely developed in birds, as distinct
elements: the bones indicated as such, by Mr. Owen, having no real or separate existence; that learned
author regards them as the neurapophyses of the fronto-neural arch, and the frontal bones as the expanded
and divided frontal spine. As growth advanced, the diploe of the coalesced frontals would begin to expand,
and the obliteration of the sutures connecting them to the facial bones, would enable the increased development
of pneumaticity to invade the frontal portions of the nasals and the median process of the premaxillary,
so as to render them tumid, and tilt their anterior walls forward, producing the marked distinction
between the cranial and mandibular segments of these elements.
The frontal protuberance culminates at a height of eleven lines above the cranio-facial hue, and seven
lines above the highest part of the parietal tract: it slopes rapidly downwards to the supra-orbital edge; its
anterior border is undefined, where the frontals coalesce with the expanded cranial portions of the nasals
and premaxillary; its posterior boundary follows nearly the posterior margin of the frontal, passing on each
side outwards and forwards from the mesial line to a point on the supra-orbital border, midway between the
notch and the post-orbital process. A broad shallow furrow traverses the median line, rendering it subbilobed;
at the posterior extremity of the groove, where the coronal fontanelle existed, is a small foramen
leading into a canal capable of admitting a fine bristle; it perforates directly the cranial parietes and opens
internally towards the apex of the cerebellar fossa, transmitting a vein from the scalp to the posterior
cerebellar sinus: the thickness of the cranium, here, amounts to eight lines and a half. Behind the foramen
is a transversely oblong band, three lines and two-thirds wide, and one Hue long; defined posteriorly by an
ungueal fissure-like groove, the angles of which extend outwards, curving backwards : its extremities are also
defined by grooves. From the foramen a venous groove passes outwards, along the posterior border of the
frontal protuberance, and externally curves slightly backwards to the aperture or notch on the supra-orbital
plate, leading to the furrow on the roof of the orbit; about half an inch external to this edge, it is joined
by a semi-circular groove which sweeps inwards, convex anteriorly, over the summit of the protuberance,
and, bending backwards, reaches the median furrow; finally diverging from it, to meet the posterior groove
at the outer and anterior angle of the band just mentioned. Where the semi-lunar grooves diverge from
each other behind, a triangular, slightly elevated tract is left on the floor of the median furrow, with its
base separated from the osseous band by a slight groove. From the convexity of the semi-lunar groove,
two others pass forwards, on the left side, to a furrow, which appears to indicate the anterior edge of the
combined frontals; in the centre it reaches half way up the frontal slope, its angle extends to the suture
between the autorbital process and prefrontal on each side.
The precise limit of the subtriangular frontal plate of the nasal, is undefined; the external limb rises
to a higher level than the internal, and is less abruptly bent on the body: on the left side, a groove
curving outwards as it retrogrades, and continuous in front with the fissure between the prefrontal and
ecto-nasal limb, indicates the outer margin of the body; the upper would form a segment of the frontal
furrow; the internal is denoted by an interrupted tissure-like groove, which may be traced upwards from
the linear impression separating the premaxillary median process, and the inner limb of the nasal, along the
upper beam of the mandible; it passes inwards as it ascends, but appears not to have come in contact with
its fellow behind, being separated in its whole extent by the termination of the premaxillary nasal process;
which ascends to touch the frontals mesially, its apex having been probably inserted into the frontal suture.
The lateral moieties of this extremity are also separated by a median fissure-like groove, which disappears
as it ascends; the left one is more tumid than the right, and its anterior bullosc extremity overhangs the
cranio-facial line; on the right side, the pneumatic diploe does not cease so abruptly, and has a tendency to
invade the median mandibular stem. This portion of the premaxillary measures six hues across its base,
resilient hinge, formed by the beak and the cranium—the cranio-facial line, at which the forehead rises
abruptly above the level of the upper mandible, it has the same form, but the anterior and posterior, as well
as the lateral edges respectively, approach to equality in length.
I t s greatest breadth, corresponding to a line drawn between the lateral rounded angles, about six lines
in front of the post-orbital processes, is three inches nine lines; anteriorly, it contracts gradually to the
supra-orbital notches, where it is three inches wide; it continues forwards for half an inch of the same
diameter, and then rapidly diminishes in width to the anterior edge, which is one inch six lines.
A little behind its greatest transverse diameter, it presents the deep temporal cmarginations, and
gradually contracts to the notches separating the mastoid from the paroccipital processes, where it measures,
transversely, two inches nine lines. The median longitudinal diameter, from the cranio-facial line to the
occipital facet, is two inches nine lines; from the anterior angle of the prefrontal to the most remote part
of the occipital aspect, is three inches two lines.
This facet is formed, behind and centrally, by the confluent, short but broad, parietals; posteriorly
and laterally by the mastoid, presenting the muscular impressions, and extending forwards, so as to enter
into the composition of the post-orbital process (which in the Emeu and Bustard is formed by a separate
element); anteriorly it is constituted by the abbreviated and coalesced frontals, which are raised by the
sudden and great expansion of the diploe into a broadly rounded, intcrorbital protuberance. The wide
semi-lunar notch, formed by their combined anterior edges, receives the bodies or frontal plates of the
nasals, which are abruptly bent upwards at an angle of 45° with the plane of the upper mandible, and
ascend high on the frontal slope to coalesce with the frontals, the sutures being obliterated: the nasals
appear to be relatively much abbreviated, and to be almost, if not wholly, separated mesially by the broad
triangular extremity of the nasal process of the premaxillary, which is wedged between them, being bent
upwards in the same peculiar manner. The vacuity left between the nasal bone and the ant-orbital process
of the frontal, on each side, is filled by the triangular body of the prefrontal; which is anchylosed externally
to the ant-orbital process, the latter advancing along its outer edge to the lacrymal groove, as already
indicated; internally it is separated from the ecto-nasal limb by a fissure, but its apex is anchylosed to the
frontal plate of the nasal. On removing the beak, the broad, flat arch is seen, formed by the prolongation
of the intcrorbital septum and the turbinated lamina passing out from it, on each side, and curving downwards
to meet the prefrontal.
I n the immature condition, the peculiar frontal protuberance of the Dodo would not be developed,
and the cranium would present a gentle slope, descending from the vertex (which is somewhat in front of
the coronal fontanelle, and corresponds internally to the most elevated part of the cerebrum), to the upper
surface of the mandible.
The profile would hence resemble that in the skull of the Calwiuis, &c, but would be relatively much
shorter, from the abbreviation of the frontal: the length of that bone, and more particularly of its orbital
segment, depends on the extent traversed by the peduncle of the olfactory nerve, ere it terminates in the
proper nerve-filaments distributed to the sense-capsule (ethmoid). It protects, as the upper segment of the
fronto-neural arch, not as the lateral moiety of a divided spine, the anterior extremity of the cranio-vertebral
tube, and is supported below by the intcrorbital septum, or centrum, of the frontal vertebra; which is
excavated and reduced to a thin vertical plate, by the fossa; for the reception of the eyeballs and their
appendages. In the Dodo, from the small relative size of the eyes, the intcrorbital septum assumes more
of the ordinary characters of a centrum, and the olfactory capsules retrograde, as i t were, and recover their
primary or normal relation to the cerebral cavity. The attentive study of this singular cranium has enabled
me to recognise the existence only of three cranial vertebra, essentially related to the tliree liigher senses.