Brai'n-cavitf/ of Ignanodon. 587 



of the anterior region cannot be determined, the walk of the 

 fossa being there deficient, owing to the presence on either 

 side of a large irregular vacuity. This opening corresponds 

 to that occurring in the same position in the skull of the 

 crocodile, where it can be seen to be an irregular interval 

 between the basi- and ali-sphenoid bones. The posterior 

 surface of the pituitary body is nearly vertical and is triangular 

 in outline. 



On each side of the infundibulum there is in the cast a 

 projecting process (HI.), which marks the point of exit of the 

 third (oculo-motor) nerve*. 



Behind the infundibulum the ventral surface of the brain 

 is furrowed by a broad transverse depression which is occupied 

 by the thick posterior clinoid ridge, and marks the division 

 between the mid- and hind-brain. 



Turning again to the upper surface, we find that imme- 

 diately behind the region occupied by the cerebral hemispheres 

 the cranial cavity undergoes a great increase in height, and 

 at the same time is much narrowed from side to side, parti- 

 cularly in its upper portion. The cast of the chamber thus 

 formed (cb.) shows a pointed elevation which rises high above 

 the hemispheres, and is so much compressed laterally that its 

 superior surface is reduced to a mere rounded ridge. On 

 each side of this prominence there is a strong ridge which 

 extends downward and forward from a little behind and 

 below its apex to a point a little in front of and above the 

 roots of the trigeminal nerve. 



This portion of the cranial cavity was no doubt occupied 

 in life by the optic lobes and the cerebellum. In the cast 

 itself, however, no indication of the form and position of the 

 former is visible, and probably, therefore, they were com- 

 paratively small, or, at any rate, did not project sufficiently to 

 impress the walls of the cranial cavity. The cerebellum, on 

 the other hand, was large, and the oblique ridges on its sides 

 above referred to, seem to indicate that it probably possessed 

 fairly developed lateral lobes (l.L) ; it is, however, possible that 

 they may merely mark tiie position of blood-sinuses. Behind 

 these lobes the cast of the cerebellar chamber is slightly con- 



* It should be noted that the projections on the cast, which are 

 marked with the numbers of the cranial nerves, do not necessarily indi- 

 cate the points of origin of those nerves, but merely the position of the 

 foramina by -which they passed out of the skull. In mo.<t instances, of 

 course, the point of exit is nearly opposite the place of ori-zin ; but in the 

 present case (that of the oculo-motor root) the nerve no doubt arose not 

 in the position shown in the cast, but from the tloor of the thalamen- 

 cephalon behind the infundibulum. 



41* 



