■G40 



PROF. R. A. DART ON TUE 



palseontological endocranial investigation up to the present 

 •time. 



Quite apart from this there is ample evidence against the view 

 that a Zeuglodont, even one so primitive as Prozeuglodon^ could 

 have given rise to Prosqnalodon. This is very evident if we call 

 to mind thoSo features pointed out in detail for Zeiujlodon sensi- 

 tivus (vide text-fig. 3) and repeated by all the Zeuglodonts. It 

 was shown in that case that the site of the insertion of the 

 olfactory peduncle in the Zeuglodonts had been drawn, as it were, 

 during atrophy under the fore brain on to the basal aspect. This 

 •contraction of the area between the olfactory peduncles and the 

 ■tuber cinereum demonstrated for us the relative atrophy of the 



Text-figure 19. , 



0/ factory pedunc/e. 



Cerebra/ /lem/sphere. 



Trige/ninus. 



Lobus mecfi'us 

 cerebelli, 

 (genera/// 

 expanc/ed.) 



Paraf/occo/aS. 



Jugo/ar 

 /easA 



^Medu//a ob/ongata. 

 Dorsal view of endocranial cast of Frosqualodon davidi Flynn. 

 About J nat. size. 



■optic chiasmatic region in Zeuglodonts, and, fortunately, the cast 

 of Prozeuglodon atrox in this crucial region is sufficiently perfect 

 to illustrate the fact that these changes were already well marked 

 in the Zeuglodonts of the -.earliest Middle Eocene deposits of the 

 Fayum {vide text-fig. 11). 



In the case of Prosqii^alodon davidi, on the other hand, we find 

 that the course of its evolution has been entirely different. 

 There is evidence here of an initial expanding of this basal area 

 between the insertion of the olfactoi-y peduncle {vide text-fig. 21) 

 and the tuber cinereum (which lies somewhere between the 

 " carotid ridges"). Crossing this wide interspace in Prosqualodon 

 ■ dixvidi we find in th.e well-defined optic chiasma the evidence of 



