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



the back of the skull much wider than the neck, and makes the 

 tympanic membrane lie much nearer the middle line than the side 

 of the skull. As it is necessary to keep this membrane exposed 

 to the outer air, any swelling of the neck will automatically lead 

 to the formation of an external auditory meatus; the beginnings 

 of this passage are seen in Arctops in a groove on the posterior 

 surface of the squamosal, just outside the end of the paroccipitnl 

 process. 



On the ventral surface Arctops has advanced over Varano- 

 saurtts in the shift forward of the great pterygoidal flanges to 

 a position far in advance of the basi pterygoid processes ; this 

 change results in a. further great enlargement of the cavity for the 

 temporal muscles, and allows the development of great pterygoidal 

 muscles with an insertion on the dorsal surface of the palate. 



This shift forward of the flanges has occurred in Bimetrodon, 

 but the conditions in Arctops differ from those in the earlier 

 form in the fusion of the posterior parts of the pterygoids and 

 parasphenoid into a massive ridged girder. This change adds 

 greatly to the strength and rigidity of the skull, but its details 

 cannot readily be explained by mechanical considerations ; the 

 most important of these is the replacement of normal basiptery- 

 goid processes by the laterally directed flat lappets which occur 

 in all Theriodonts. 



In the palate itself the more important changes are the 

 development of a median groove, a necessary preliminary to the 

 establishment of a secondary palate, which is brought about by 

 that change in the dentition involving the development of a 

 "step" between the maxillary a,nd incisor teeth which has been 

 discussed under Dimelrodon, and the extreme posterior position 

 of the hinder ends of the posterior nares ; this latter change 

 is itself probably to be associated with the incipient secondary 

 palate, leading as it does to a longer air-passage whose posterior 

 end is not so easily closed by the presence of food in the anterior 

 part of the mouth. 



Another advance in Arctops is the more vertical position of the 

 occiput. 



Gorgonops is in some ways as primitive as Arctops, with which 

 it shares very broad parietal and interorbital regions and a 

 square sectioned snout. The orbit faces laterally in both forms, 

 and each has a remarkably broad interparietal, a feature in which 

 they resemble Deinocephalians. 



Gorgonops has a pair of large frontals, which are cruciform in 

 plan exactly as in the later Pelycosaur. 



The prefrontal is a large bone which overhangs a well-marked 

 depression on the preorbital surface. 



Gorgonops shows advances over Varanosaurus which, so far as 

 the parts are known, include all those which occur in Arctops, 

 with the following additions :— The external nostril in Gorgonops 

 is much complicated by the great development of the septomaxilla 

 and the foramen behind it. This foi^amen, first recognised by 



