318 Mr. D, ]\r. S. Watson on 



Tliis appears to be a very primitive cLaractcr. 



Ill Sphenodon and in all ordinary reptiles with which 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, if 

 one exists in the diiod sknll as in Sjjhenodon and Chelunia, 

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



In the skull of Tlieromus, 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 rednction of the qnadrate, and of all the bones which 

 lie below the ojiisthotics, 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 

 j)ossible that gradual enlargement of the brain which is the 

 dominating factor of the evolution of mammals. Only in tliis 

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

 or level with the base of the brain. 



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

 pro-oticnm, for the last branch of the trigeminus. Jnst 

 in front of this notch lie two low processes, presumably 

 processi anteriores infcriorcs [)ro-otieornm. Still fnrther 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 periotic as they are in Diadnnoiluii and Cynoynathus. 



From their upper surfaces just after they leave tiie basi- 

 sphenoid rise small processes, which are undoubtedly the 

 bases of the columellae crauii, here only very short antero- 

 posterioily. In front of them is a smooth wide groovr, 

 which probably marks the })oint of exit of the II., III., W ., 

 Y.', and VI. nerves. 



'V\\c anterior end of the basisphenoid ends in a very sharp 

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

 spheuoidal rostrum. 



As Seeley states, the ([uadrate appears to be recog- 

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

 well enough shown to be describable. 



The squamosals show the presence of a small post-temporal 

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



W 



