Skull, Brain, dec. of Diademodon. 221 



front of the tabulare, which is itself overlapped by the 

 squamosal. 



The squamosal has a large articulation with the end of the 

 opisthotic ; it sends a process inward over the post-temporal 

 fossa to cover the front face of the tabulare and part of the 

 parietal. It has also a connection with the pro-otic by the 

 special process, already described, which covers the venous 

 canal between the pterygo-paroccipital and post-temporal 

 fossae. 



In my former description I held the thin plates of bone 

 which cover the anterior parts of the brain and extend back- 

 wards to the quadrate in front of the tympanic cavity to be 

 parts of the pterygoid. The present specimen shows con- 

 clusively that that view was wrong, and that Dr. Broom was 

 right in considering the whole to be a separate bone — the 

 epipterygoid. 



The suture separating the epipterygoid from the pterygoid 

 runs obliquely from the notch in front of the columella 

 cranii to the lower surface of the boues below the foramen 

 for the V. nerve and to the outside of the opening for the 

 internal carotid artery. This condition explains the fact 

 that the relation of the pterygoid flange of the quadrate and 

 the quadrate flange of the epipterygoid is the reverse of that 

 which always occurs between the pterygoid llange of the 

 quadrate and the posterior ramus of the pterygoid. 



The process by which the reduction of this posterior ramus 

 of the pterygoid and development of the epipterygoid takes 

 place is clearly illustrated by many " Therocephalian " skulls, 

 where the epipterygoid has an articulation with the basi- 

 sphenoid and sends a long process back along the upper 

 border of the posterior ramus of the pterygoid. 



The epipterygoids extend forward to the middle of the 

 orbits, their lower borders being closely approximated during 

 much of the anterior part. 



In transverse section just behind the postorbital bar the 

 upper edge of the epipterygoid is clasped between the frontal, 

 which sends a flange down its inner side, and the postorbital, 

 which covers a good deal of its outer surface. 



It seems to be certain that the whole of this long anterior 

 extension is really epipterygoid and that there is no orbito- 

 sphenoid. 



The structure of the bony labyrinth is very well shown on. 

 the right side of the type-skull of D. entomophunus ; the 

 actual semicircular canals, however, have not been cleared, 

 but are very clearly seen as casts in the type-specimen of 

 Nythosaurus larvatus, which agrees closely in structure. 



