642 



TROF. E. A. DART ON TIJE 



is also off the main line of Cetacean evolution would probably be 

 indicated by an examination of the endocranial casts with regard 

 to these details. 



The conformation of the district between the olfactory 

 peduncles and the chiasma — in fact, the whole " mamma-like '* 

 appearance of the antero-ventral portion of the fore biuin in 

 Frosqualodon davidi, contrasting as it does with the same region 

 in the Zeuglodonts — is consonant with the homologous region in 

 true Oetacea. A comparison of this region with the ventral 

 aspect of the brain in the foetal Ifonodon or adult Kogia (vide 

 figures in Elliot Smith's account, 1903) demonstrates the truth 

 of the conception that the ancestors of true Oetacea went through 



Text-figure 21. 



, Olfactory pedunc/e- 



Carotid ridge. 



MiddJe cerebral 

 vessels. 



Trigeminus, 

 (Mandibular 

 • dii^ision.) 



Parafloctuh 



Medulla oblongata. 



Ventral view of endocranial cast of Frosqualodon davidi Flynn. 

 About \ uat, size. 



a stage of " expansion of the basal regions of the brain " (witness, 

 the "desert region" of Broca in Oetacea) not displayed by 

 Zeuglodonts but evident enough in Frosqualodon. 



What this expanded "desert region " or tuberculum olfactorium 

 in whales exactly signifies, we are not as yet able to definitely 

 state. Since this area is the site of the palteostriatal cortex 

 overlying the palaeostriatum (Dart, 1920) it follows that the 

 persistence of a large " desert region " is dependent on factors- 

 other than olfactory, i. e., factors which have determined the size^ 

 of the palseostriatura itself. The consideration of what these 

 factors actually may be lies outside the scope of this article, but 

 the essential point affecting ouv argument here is that these: 



