BEAIN OF THE ZEUGLODONTID^. 



631) 



The dorsal view corroborates what we have ah-eady detected 

 from the lateral aspect. The wide expanse of the "area crescens"^ 

 cerebelli is the most obtrusive feature and after that the 

 expanding cerebrum. The double olfactory peduncle is clearly 

 seen but no Gasserian ganglion is visible (text-fig. 19). 



The posterior view is instructive in revealing an enlarged oval 

 transverse section of the medulla oblongata and still further 

 illustrates the nature of the cerebellar expansion (text-fig. 20). 



Perhaps the most informative of all is the ventral view, because 

 we are able to i-ecognise the true size of the trigeminus (which 

 is quite large), the relatively enormous width of the base of the- 

 brain, the marked development of the optic chiasma, and the- 

 relative atrophy of the olfactory apparatus. There is an entire 

 absence in this brain of anything corresponding to the tuber- 

 culum olfactorium or " intertubercidar sulcus " of the Zeuglo- 

 donts, but we find a ridge on either side of the middle line medial 



Text-figure 18. 



Lob us medius 

 ^ cerebe//i. 

 (generslfy expanded.) 



Olfsctory 

 peduncle. 



Trigsmintjs. 



/eash. 

 M/W/e cerebri/ vesse/s. 



Lateral view of eiidocranial cast of Prosqualodon davidi Flynn. 



About J iiat. size. 



to the trigeminal region which may be due to the carotid artery 

 and accompanying venous sinuses. The tuber cinereum in not 

 apparent in the cast, but must lie between these ridges (text- 

 fig, 21). 



Prosqualodon davidi Flynn comes from the Miocene deposits of 

 Tasmania, and the characters which it presents linking it to the 

 Zeuglodonts might well be interpreted as illustrating its origin 

 from the Zeuglodont family if our information were not so- 

 complete as it is now. 



The actual bulk of this endocranial cast is approximately 

 750 c.c, i.e., not quite so great as that of Zeuglodon iniermedius.. 

 This fact itself is sufficient to show that the Eocene form which 

 gave rise to Prosqualodon must have possessed a brain -capacity 

 very much less than that of Prozeuglodon atrox or of Zeuglodon 

 iniermedius. This follows from the well-known " law of increasing 

 brain weight" put forward by Marsh and supported by all 



