•038 rnoF. n. a. daut on the 



whereas the Sirenia have persisted as " living fossils," despite 

 their humble grade of intelligence, the Zeuglodontidas have long 

 since disappeared. 



This disappearance is to be correlated with the fact that, 

 whereas the Sirenia chose an eminently retired and sluggish 

 mode of existence, the Zeuglodontidse were more active. They 

 came into more direct conflict with other marine forms and, 

 specialised as they were, did so. at a disadvantage and were over- 

 whelmed in the struggle for existence. 



The present study therefore indicates that the scepticism of 

 Marsh and Fraas was entirely justified and that Winge (1921) 

 is incorrect in regarding the Zeuglodontida} as ancestral to 

 Cetacea. The oingin, dispersal (into so many strikingly different 

 forms and all over the globe), and disappearance of this group 

 geologically is a demonstration of the conception put forward by 

 Marsh (1877) that "In every primitive type which was destined 

 to survive many geological changes there seems to have been a 

 tendency to throw off lateral branches which become highly 

 specialised, and soon die out, because they are unable to adapt 

 themselves to new conditions." 



7, Comparison of Zeuqlodont and Prosqualodont Brains. 



The above paper Avas practically ready for the press when 

 Prof. Elliot Smith received from Tasmania, through the great 

 courtesy of Prof. Flynn of the Zoological Department in the 

 University at Hobart, a splendid cast of the cranial cavity of 

 Prosqualodon davidi which he has recently described. Plaster 

 replicas of the skull from which this cast was made were pre- 

 sented by him to the Bi-itish Museum of Naturar History and 

 to the Zoological Department of University College, London. I 

 have had the privilege of studying these casts and the endocranial 

 cast. 



The lateral view of this cast (text-fig. 18) reveals an astound- 

 ing likeness to the Zeuglodont endocranial cast. We meet with 

 the same cerebellar enlargement, expanding forwards over the 

 cerebrum, and quite an enlarged trigeminus. 



It is obvious that such striking similarities would not exist in 

 the absence of some close relationship between these forms, but 

 the data will reveal that this relationship is not a filial one as it 

 has frequently been conceived. 



It is to be noted that the parallocculus is I'ecognizable laterally, 

 but is submerged by a widely and generally expanded cerebellum. 

 The cerebrum is emancipating itself from the cerebellar growth 

 as its great height posteriorly shows. Anteriorly the rapidly 

 dwindling olfactory apparatus is attached in a " nipple-like " 

 fashion to the cerebrum and considerably ventral to it is a 

 well-marked optic chiasma. The middle cerebral vessels reveal 

 a course comparable with that seen in the Zeuglodonts and 

 posteriorly there is a well-marked jugular leash. 



