80 OSTEOLOGY [Parr I. 
gular surface is broken by the projection of the anterior wall of the tympanic cavity, it slopes backwards 
and outwards; its upper margin extends to the post-temporal process of the mastoid ; the lower is horizontal 
and directed to the inferior angle of the paroccipital process. 
The roof of the orbit is formed by the frontal, and by the ali-sphenoid, for a short space anterior to the 
temporal fossa; a lne drawn from the optic foramen, through the foramen ovale, to the posterior tympanic 
articular facet, will indicate the lower margin of the ali-sphenoid. The inferior boundary of the mastoid corres- 
ponds to a line drawn from the notch between it and the paroccipital process, through the superior tympanic 
aperture to the inner angle of the root of the post-temporal process, where it comes into apposition with the 
external border of the ali-sphenoid; thence the suture passes forwards, inclining upwards to the post-orbital 
process. The division between the sphenoid and the ex-occipital follows the course of the canal which 
runs behind the fenestra ovalis; its upper angle is anterior to the inferior tympanic articular surface, 
which is developed on the ex-occipital ; below it passes internal to the Hustachian tube, cutting through the 
elliptic fossa, common to the foramen caroticum and f. lacerum posterius, and lastly bends transversely 
inwards, intersecting the foot of the basilar protuberance. The diminished area of the interorbital septum, 
which is only about six lines in diameter, is remarkable, and is due to the small size of the eyes, which are 
amply protected by the great outward projection of the roof of the orbit posteriorly. The proper septum is 
reduced to the small space, intervening between the base of the olfactory fossa and the interval separating 
the foramina optica, in the antero-posterior diameter ; it is encroached on above by the expanded frontals, 
and below by the inflated rostrum. From the abbreviation of the cranium, and consequent shortening 
of the frontal, the orbital vault is relatively very small ; it is bent down abruptly anteriorly, nearly at right 
angles, and, as it were, pressed backwards ; the angle of flexure corresponding to the supra-orbital notch, 
from which the roof increases in breadth as it retrogrades obliquely downwards. 
A line drawn from the supra-orbital to the temporal notch would cut off an elongated triangular 
segment ; the hypothenuse corresponding to the convex, thick, and rough supra-orbital margin, and the 
base to the post-orbital process. The great breadth of the interorbital region, which is continued back- 
wards diminishing very gradually to the mastoid notch, and the flattening down, as it were, of the roof of 
the orbit behind the eye; together with the great elevation of the forehead above the surface of the man- 
dible, and its contraction in front of the supra-orbital notch, are remarkable peculiarities m the head of 
this extinct form. The roof of the orbit is arched transversely, but more flatly concave longitudinally than 
the anterior portion of the orbital vault; the greatly increased expansion of the diploé of the frontal inter- 
nally, causes its surface to descend rapidly into that of the interorbital septum ; while from the retrogression 
of the olfactory fosse, the anterior wall slopes very gradually backwards. A line drawn from the inferior 
extremity of the prefrontal to the post-orbital process, measuring one inch and seven lines, ascends 
obliquely backwards at an angle of 45°; and a plane extended inwards from it to the optic foramen would 
limit the orbit posteriorly and inferiorly. The depth of the vault from the supra-orbital notch, is one inch 
one line. 
The foramen opticum, situated at the apex of the triangularly pyramidal orbital fossa, is equidistant 
from the anterior and posterior surfaces of the cranium, and from the supra-orbital and mastoid notches ; 
its circular contour is notched above by a vascular groove, and its relatively small diameter is two lines and a 
half. Its floor is four lines and a half from the basilar surface, and its roof one inch ten lines and a half 
beneath the highest point of the frontal protuberance. The anterior edges of the foramina of opposite sides, 
are separated by an interval of six lines and two-thirds, corresponding to the broad posterior border of the 
interorbital septum, which is convex transversely, and concave vertically. 
The ant-orbital foramen, for the transmission of the ophthalmic branch of the trigemmal nerve and 
