736 MR. D. M. S. WATSON ON 
much excavated in the middle, and its lateral borders rise to the 
opisthotics. The upper surfaces of the processes spoken of above 
are deeply grooved, and the grooves from them pass on to the body 
of the bone, forming a depression stretching right across it. In 
front of this is a ridge which marks the posterior border of the 
pituitary fossa, whose surface is not very well preserved in any 
case. 
Text-figure 1. 
BSP Vest. BOc. 
Procolophon trigoniceps. 
A. Base of skull and otic region viewed from below and the right side. X 2. 
B. View of lett side of brain-cavity. 2. 
B.Oc., Basioccipital ; B.Pr., Basipterygoid processes ; B.Sp., Basisphenoid ; 
Ev.Oc., Exoccipital ; F.Jug., Foramen jugulare; #.V., Foramen Vidii? ; 
Fen.Ov., Fenestra ovalis; Op.O., Opisthotic; P7.0., Pro-otic; Vesti., 
Vestibule. 
The exoccipital is a bone of medium size, articulating with the 
upper surface of the basioccipital. It is comparatively long, and 
is pierced at about the middle of its length by two small foramina 
for the twelfth nerve, which he just above its lower border. 
These two openings soon join and open ina single foramen on 
the outside of the cranium. The bone extends outwards for a 
little distance with its upper part in contact with the posterior 
surface of the paroccipital, from which bone it is separated below 
by the large, round foramen jugulare. The exoccipital forms the 
