318 Ml-. D, M. S. Watson on 



This appears to be a very primitive character. 



In Sphenodon and in all ordinary reptiles with wliicli I am 

 acquainted the inner ear lies to the side of the brain-cavity 

 and not at all below it. The opening into the inner ear, il: 

 one exists iu the dried skull as in Sphenodon and Chelonia, 

 forms a great hole in the side wall of the brain-case. 



In the skull of Theromus, however, this opening faces 

 downwards and is largely on the lower surface of the brain- 

 case, so that the inner ear lies below the level of the bottom 

 of the brain. 



This remarkable character probably depends on that 

 great reduction of the quadrate, and of all the bones which 

 lie below the opisthotics, which occurs in and characterizes 

 the Therapsids. What was the origin of this reduction we 

 do not know, and in the present state of our knowledge of 

 early Therapsids it is useless to speculate ; but the change in 

 the position of the ear was of great importance, for it rendered 

 possible that gradual enlargement of the brain which is the 

 dominating factor of the evolution of mammals. Only in this 

 skull and in mammals is the inner ear known to lie below 

 or level Avith the base of the brain. 



In advance of the auditory region is a notch, the incisur<i 

 pro-oticum, for the last branch of the trigeminus. Just 

 in front of this notch lie two low processes, presumably 

 processi anteriores inferiores pro-oticonuu. Still further to 

 the front lies the small deep pit which Seeley regarded as a 

 carotid canal, a view which may very possibly be correct : 

 it might, however, have lodged a small pituitary process. 



Just in advance of this region the posterior rami of the 

 pterygoids pass out from the sides of the basisphenoid 

 towards the quadrates. They are not supported by a process 

 from the jjcriotic as they are in Diademodon and Cynognathus. 



From their upper surfaces just after they leave tlie basi- 

 sphenoid rise small processes, which are undoubtedly the 

 bases of the columellse cranii, here only very short antero- 

 posteriojly. In front of them is a smooth wide groove, 

 which probably marks the point of exit of the IL, III., IV,, 

 V.', and VI. nerves. 



The anterior end of the basisphenoid ends in a very sharp 

 higl) ridge, which is most probably the beginning of a para- 

 sphenoidal rostrum. 



As Seeley states, the quadrate appears to be recog- 

 nizable on the right side ; it is comparatively large, but not 

 Avell enough shown to be describable. 



The squamosals show the presence of a small post-temporal 

 fossa, of a wide and shallow V-shaped notch separating the 



