218 Mr. D. M. S. Watson on the 



small Diademodon skull which fits accurately and is a part 

 o£ Seeley's type-specimen of Diademodon entomophonus, 

 collected some twenty-five years before. It is interesting to 

 note that this long exposure cannot at the most have removed 

 more than half a millimetre from the exposed broken face. 

 The matrix is a light green and red mottled cornstone, which, 

 though hard, leaves the bone cleanly. I have been able to 

 completely clean the whole brain-cavity behind the exit of 

 the fifth nerve, and also the inner ears of both sides, that on 

 the right more completely. 



Broken surfaces, some of which have been polished, enable 

 one to get a clear idea of the relation of the membrane-bones 

 of the back of the skull to the otic capsule. 



The following description should be read in connection 

 with my former description of the Diademodon skull. 



In front of the small round foramen magnum the brain- 

 cavity expands considerably, forming a large cavity much 

 higher than wide. The floor is slightly concave, and in front 

 is sharply depressed to form the pituitary fossa, whose floor 

 is extremely narrow. The pituitary fossa is flanked on either 

 side by the processi inferiores anteriores of the prootics. 



Lying on the extreme' back of the skull just above the 

 condyle is the small notch, which is probably for a blood- 

 vessel. 



In advance of this at the sides of the base of the brain- 

 case, where the lateral walls turn sharply upwards, are two 

 small foramina lying one in front of the other. These open 

 into the deep pit which was described as the foramen jugu- 

 lare, the posterior at the extreme back of the pit and the 

 other on the inner wall just in front of it. These foramina 

 are undoubtedly for the twelfth nerve. 



Above the posterior hypoglossal foramen is a small 

 foramen opening upwards in the lateral wall of the skull. 

 1 have not been able to trace the course of this opening, but 

 think it probably merely venous ; it is certainly not a nerve- 

 exit. 



In front of these foramina is a large hole of irregular 

 shape in the wall and base of the brain-case. The posterior 

 part of this is the foramen for the IX., X., and XL nerves ; 

 the rest is the opening into the vestibule. 



Immediately in front of the opening lies a very small 

 canal — the aqueductus fallopii — which passes straight down- 

 wards through the prcotic. Above the internal auditory 

 meatus is a large deep fossa subarcuata : this passes back- 

 wards and'outwards, and has a smoothly rounded end ; it is 

 separated from the ear by a thin horizontal flange of bone, 



