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



but their direction is then changed as they form the slender 

 snout. The skull is much wider than high. 



In the Brain-case. 



[The structure of the anterior face of the pro-otic and supra- 

 occipital is not known in Varanosaurus and other regions are not 

 very well shown. The following account of the Pelycosaurs is 

 based on Deiopeiis and Dimetrodon.^ 



In Pelycosaurs the basioccipital is thick, ending in a large 

 rounded condyle. The large fenestras ovales are placed on the 

 bottom of the skull, far out at the side oi' the deep \\' ell-developed 

 tubera. The paroccipital process is short and slender, supporting 

 the squamosal and touching the tabular ; it lies well above the 

 lower surface of the skull. The pro-otic is small, its anterior face 

 in no way overhanging the notch for the fifth nerve. The supra- 

 occipital is entirely plate-like, not forming a roof over the brain 

 in advance of the Yth nerve. The basisphenoid is massive, 

 forming a sloping floor to the posterior part of the brain-case. 

 It bears definite Sphenodon-\i\s.e basiptei-ygoid processes, anteriorly 

 it is in Varanosaurus continued forward by a long channel -shaped 

 parasphenoid. 



The parietal does not form any part of the side-walls of the 

 biain-case. The epipterygoid is a slender rod of circular section. 



The whole brain-cavity is very small in comparison with the 

 size of the skull. 



In Diademodon, on the other hand, the basioccipital is small 

 and plays at most a subsidiary part in the pair of occipital con- 

 dyles. The small fenestras ovales are placed on the bottom of the 

 skull, not very far separated. Basisphenoidal tubera. are repre- 

 sented merely by the edges of the triangular lower face of the 

 basisphenoid. The paroccipital is a long powerfid pi'ocess sup- 

 porting the squamosal and touched by the tabular; it lies on tlie 

 lower surface of the skull. 



The pro-otic is large, being carried forward by a. great pi'ocess 

 which completely overhangs the trigeminal foramen. 



The supraoccipital is produced forwards by two wings, which 

 cover and form side-walls to a great deal of the bi'ain-cavity in 

 advance of the Vth nerve. 



The basisphenoid is a small bone forming a nearly horizontal 

 floor to the brain-case. It has small lateral basipterygoid pro- 

 cesses with the pterygoids attached to their flat lower surfaces ; 

 anteriorly it is carried forward by a slender process which reaches 

 the palate and there spreads out into a broad vomer in the roof 

 of the posterior part of the naso-pharyngeal ducts. 



The parietal forms a good deal of the side-wall of the brain- 

 case. The epipterygoid is a flat plate forming the side-wall of 

 the brain-case for some dista,nce and articidating with the anterior 

 edges of the pro-otic. The brain-cavity is I'elatively very large. 



The ear of a Pelycosaui-, so far as can be inferred from the 

 bone which housed it. lies low down on the side of the brain-case, 



