the Skull of DiademoJon. 325 



This seems to mc good evidence that the tympanic is not 

 derived from a hone whose main if not its only function is 

 tlie protection of the nndcr surface of the ear. 



I am therctore unable to accept the identification of the 

 problematical bcnic in Cynodonts called by Broom tym[)anic 

 with the bone of the same name in Mammals. If a mamma- 

 lian homologue must be found for it, the cntot\mi panic, 

 which stretches from the tympanic to the basisphenoid in 

 Tupnia, seems the most suitable ; but as this bone does not 

 seem to be of wide distribution in the Mammalia, it is 

 probably a neomorph. 



The resemblances of the Cynodonts, and, indeed, of 

 T!iera[)sids generally, to ]\Iammals have been commented on 

 by all authors from the time of Ovven^s original description 

 till to-day, and almost all recent descriptions of their 

 remains have been written from the mammalian standpoint. 

 The preceding description is one of the few exceptions, and 

 it was not nntil I had spent many days in fruitlessly trying 

 to identify the various foramina by comparison with marsu- 

 pial and iuscctivore skulls that I commenced a comparison 

 with other reptiles. 



The resemblances of Therapsids to Mammals are : — 



1. The presence of a single zygomatic arch formed by the 



squamosal and jugal, (Owen, Sceley, Broom.) 



2. The division of the teeth into incisors, canines, and 



cheek-teeth. (Owen, Seeley, Broom.) 



3. The mode of insertion of the teeth. (Owen, Seeley.) 



4. The presence of a secondary palate in Cynodonts. 



(Seeley, Broom.) 



5. The presence of a median vomer (developed from the 



parasphenoid?). (Broom.) 



6. The great reduction of the quadrate and the bones of 



the lower jaw except the dentary. (Seeley, Broom.) 



7. The presence of a long distinct external auditory 



meatus. (Gregory.) 



8. The double occipital condyle of Cynodonts. (Seeley.) 



9. The general resemblances in the vertebral column. 



(Seeley.) 



10. The very mammalian form of the scapula and pectoral 



girdle. (Seeley, Broom.) 



11. The build of the humerus. (Owen, Seeley, Broom.) 



12. Tlie presence of an olecranon process. (Seeley). 



13. The structure of the carpus. (Seeley, Bardelebcn, 



Broom.) 

 Ann. i£' Mag, lY. Hist. Ser. 8. Vol. viii. 22 



