202: 



PKOCEEDIBTGS OF THE GEOLOGICAL SOCIETY. [Jan. 25, 



In the middle line, from behind forwards, the basicranial axis exhibits 

 first the under surface of the occipital condyle (of), 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 posi- 

 tion 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 toge- 

 ther form a triangular base, from the front of which ascends a pro- 

 jecting four-sided rod (bjys), 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. Eather more than 

 the posterior half of this surface of the rod ig hollow longitudinally 

 and transversely. In front the hoUow contracts, and is followed by 

 a median ridge, of which the base only remains. From the begin- 

 ning 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 (ii) 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 hypo- 

 physial 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 trans- 

 mitting 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 pre- 

 sphenoid, 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 sur- 

 face 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 



