213 



PrO 



with the corresponding process of the opisthotic forms the fenestra 

 ovale. 



The Exoccipital-epiotic is a large plate of bone whose lower 

 border has a notch forming the top of the foramen magnum and brain 

 case. It articulates below with the exoccipital-opisthotic and pro-otic, 

 its lower lateral border being wedged in between the lateral expansions 

 of these bones. In Endothiodon it is pierced by a rather large ductus 

 endo-lymphaticus, but in the other types this issues over a notch 

 in the very large internal auditory meatus. The structure of the back 

 of the skull described above 

 is found with only slight 

 modifications in all orders 

 of Therapsids, and appears 

 to be emi nently characte- 

 ristic of the group. 



The chief general cha- 

 racteristic of the Therapsid 

 ear is that the vestibule lies 

 to a great extent below the 

 level of the base of the brain, 

 the Anomodonts are peculiar 

 in the carrying down of the 

 fenestra ovale to the bottom 

 of a long projection of the 

 vestibule. The internal ear 

 is best shewn in the speci- 

 mens of Dicynodon and Kan- 

 nemeyeria. I have been 



helped in interpreting it by comparison with the ear of Diademodon 

 which I now know in some detail. 



The joint foramen jugulare and internal auditory meatus is a 

 large hole of irregular shape in the side and bottom of the braincase. 

 The foramen is separated from the vestibular cavity by a ridge on 

 the outer side of the cavity, which, some distance below the level 

 of the bottom of the brain, reaches across so as to completely se- 

 parate the canal for the nerves from the vestibule, the upper end of 

 this partition has a rounded notch which is conclusively shewn by 

 a comparison with the more ordinary ear of Diademodon to have 

 formed the lower border of the fenestra rotunda. The inner side 



Fig. 3. The same specimen as ttie pre- 

 ceding figures in sagittal section medial aspect. 

 XI. Reference letters as before. 



