50 MR. D. M. S. WATSON ON THE 



of its suture with the pro-otic. Above and in front of the 

 termination of this suture the basisphenoid is still continued 

 upward as a slender process, whose upper margin is the lower 

 border of the great venous notch and whose lower margin meets 

 its fellow in an opeu median suture below the brain, immediately 

 in advance of the hypophysis. The rostral part of the basi- 

 sphenoid is a vertically placed plate arising from the body of the 

 bone and separated from the upper parts, just described, by a 

 deep narrow notch, the open side of the pituitary fossa. From 

 its sides arises the thick flat expansions, which are the basi- 

 pterygoid processes. These incline downwards at the back at an 

 angle of about 45°, and whilst their dorsal surface is sharply 

 separated from the vertical face of the medial lamina, which lies 

 above them, their ventral faces pass smoothly down to form a 

 blunt ridge on the lower surface of this part of the basisphenoid. 



The parietal is composed of a plate of bone lying on the roof of 

 the skull with an almost plane dorsal surface. Its postero-lateral 

 corner is drawn out into a long process, which passes backwards 

 to touch the extreme tip of the squamosal. The posterior edge 

 of the whole bone is in contact with the interparietal towards the 

 middle line and with the tabular laterally. By far the greater 

 part of the outer margin of the parietal is in contact with the 

 postorbital, which completely excludes it from participation in 

 the margin of the temporal fossa. From the lower surface of the 

 parietal a powerful ridge is developed, which marks the side-wall 

 of the brain-case. Posteriorly this ridge just touches the anterior 

 end of the supraoccipital. Immediately in front of this bone it 

 has a suture with the epipterygoid ; further forward its lower 

 edge is free, but gradually declines, until at or about the front 

 end of the parietal it vanishes. The lower surfaces of the pre- 

 parietal and frontals form the roof of the brain-case in this 

 region, and the lower surface of the anterior part of the brain 

 is supported by an ethmoid ossification. This is a thin hemi- 

 cylinclrical shell of bone with a rib along its ventral surface in the 

 middle, which indicates that it rested on a deep median septum 

 now broken away and lost. 



The posterior end of the ethmoidal cavity is widely open. The 

 opening of the anterior end is much contracted and lies close up 

 to the skull-roof. 



The floor of the cavity close to its anterior end is perforated 

 by a pair of large oval foramina, which face downward. These 

 are separated only by a narrow septum and must be for the optic 

 nerves, which hence had a remarkably long intracranial course. 



The epipterygoid is only represented by its upper end, which, 

 though narrow a,ntero-posteriorly, is thin. It has a suture with 

 the parietal and with the front end of the supraoccipital, the 

 latter connection being of considerable morphogenetic importance. 



There is a medium-sized foramen for the Xth nerve, opening 

 downwards and backwa,rcls below the exoccipital well above the 



