Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/328

202 In the middle line, from behind forwards, the basicranial axis exhibits first the under surface of the occipital condyle (ot), which is followed by a constriction .8 long, just mentioned. To this succeed a pair of large, blunt, conical swellings (bs), the obtuse summits of which point downwards, outwards, and backwards. They are extensions of the basisphenoid, and they form with the condyle three points on which the skull rests when its base is placed on a flat table, in which position the floor of the cranial cavity is nearly parallel with the plane in which the points lie. Behind, a deep narrow cleft separates the right and left basisphenoidal swellings ; in front they join and together form a triangular base, from the front of which ascends a projecting four-sided rod (bps), three sides of which are seen at the under surface of the skull, while the fourth is within (in the floor of the cranial cavity). This rod measures at the under surface of the skull 3.9 inches long. It rises obliquely forwards, making with the floor of the cranial cavity an angle of about 40°. Its under surface behind is nearly 1.2 inch broad ; it tapers slightly forward, so that at the distance of 2 inches it has diminished .3 inch. Rather more than the posterior half of this surface of the rod is hollow longitudinally and transversely. In front the hollow contracts, and is followed by a median ridge, of which the base only remains. From the beginning of this ridge forwards the rod rapidly narrows ; and here the lower border is fractured, making it probable that there was a thin onward production of bone between the orbits.

Along the sides of this axial rod are arterial and nerve-foramina. The first and second pair of these, reckoned from behind (c, c'), are respectively the posterior and anterior apertures of a right and left canal tunnelling the basisphenoid and crossing the bottom of the hypophysial fossa, which is the course taken by the internal carotid artery in reptiles. The third pair of foramina (n) are the outlets of two short canals passing forwards and outwards from a transverse depression in the floor of the cranial cavity, situated directly in front of a low transverse ridge, which has the same relation to the hypophysial fossa that the tuberculum has to the sella turcica in the human skull. This transverse depression corresponds, then, to that which in the human skull lodges the optic commissure, and doubtless here subserved the same purpose, the canals continued from it transmitting the optic nerves to the orbit. It follows that the front half of the ascending part of the basicranial axis as it lies between the optic nerves and in front of the tuberculum sellse contains the presphenoid, the hinder half consisting of the basisphenoid. No traces of the junction of the presphenoid and basisphenoid, nor of this with the basioccipital, are discernible in a longitudinal section of the skull ; the sutures which once existed between these originally separate bones have completely disappeared.

At each side of the presphenoid, above the optic canals, in the under surface of the skull is a smooth triangular space, the upper surface of which contributes to form the flat floor of the anterior fossa of the cranial cavity. Its relations show it to be the orbito-sphenoid (fig. 2, os). The hinder border of that part of it appearing at the