70 Mr. D. 11. S. Watson on the 



The posterior part of the pterygoid, which alone remains, 

 has its inner margin in contact with the parasplienoid and 

 bas^isphenoid tor a considerable distance. At the back it is 

 connected witii tlie basipterjgoid process; underlying it, and 

 havin<r a dorsal ridge which passes round its end and is con- 

 tinned towards the quadrate region. On the left side of the 

 skull this dort=al ridge is separated by a crack, which,! think, 

 is not a suture ; it may conceivably, however, be one, in 

 which case tlie dorsal portion would be epipterygoid. In 

 front the two pterygoids ajipear to meet under the para- 

 sphenoid ; they then separate to form a very small inter- 

 pterygoid vacuity and periiapsmect again in front. On each 

 side of the vacnit}' is a powerful process, deep but rather 

 narrow antero-posteriorly. The outer half of this process 

 is covered in front by the transverse bone, which rises almost 

 vertically to the maxilla. 



The ethmoid is only exposed in section on the front of the 

 specimen ; it forms a thin vertical septum articulating below 

 vith the top of tbe parasplienoid, and opening out above into 

 two wings so as to form a groove for the olfactory nerves. 

 It does not appear to reach the top of the skull. 



AYhere corresponding parts are present, this specimen, the 

 type specimen of Gonjonops torvus, and three skulls of 

 J^ct/mnosvcJius ic/iaitsi agree closely in all their important 

 structural features. 



Some General Considerations concerning the 

 Structure of Therapsid Skulls. 



Homologies of Bones. 



InterparietaL — The interparietal occurs in all satisfactorily 

 known South African TJierapsid skulls. Jt seems to be in all 

 particulars similar to the mammalian bone of the same name. 

 It is to be remembered that in ontogeny this latter bone is 

 paired, so that it is in every way probable tiiat it was arrived 

 i;t by the fusion of two bones. Ihe bone agrees closelj witii 

 the postparietals of the Stegocephalian skull, lying as it does 

 dorsal to the su])raoccipital and epiotics and behind the 

 parietals. The Stegocephalian postparietal is a dermal bone 

 which covers the upper end of the epiotic ; in later types it 



