632 



PROF. n. A. DAUT ON TUK 



a degenerating condition in this later Zeuglodont type. When, 

 we i-ealise that ;>art joassw with this decrease ii:i volume there 

 is an actual inci-ease in the lateral thrusting of the brain-mass 

 and with it of the Gasserian ganglia, and a relative flattening 

 of the mass of medulla oblongata, it seems clear that this 

 degeneration is priniarily trigeminal in nature. 



4. c. Zeuglodon osiiiis Dames. (M. 10228.) 



A comparison of the lateral aspect of the cast of Zeuglodon 

 osiris from the upper beds of the Middle Eocene with the pre- 

 ceding forms is very striking. One gathers the impression that 

 the brain-mass is shrivelled and shrunken, a conception that is 

 strengthened by examination of the dorsal and posterior views. 

 If we take the features seriatim we find first of all that the 

 Gasserian ganglion is relatively small as compared with the other 

 forms, but its stranded position (laterally to the cerebrum — see 

 dorsal view) shows that this creature is descended from a type in 



Text-figure 15. 



Cerebral hemisphere 



Lobus medi'us cerebelli,- 



Paraflocculus. 



Trigeminus. 



Gasserian ganglion. \^^'---^Li 



Medulla 

 oblongata. 



'Jugular leash. 



Lateral view of endocranial cast of Zeuglodon osiris Danies. M. 10228. 

 About ^ iiat. size. 



which the trigeminal apparatus was of a much more imposing 

 nature. The " gouged out " cerebrum leveals a degenerate con- 

 dition of this organ. Obviously the olfactoiy region has become 

 even more atrophied than in the other known Zeuglodonts. 

 Despite the opportunity offered by this atrophy anteriorly and 

 the <iiminishecl size of the cerebellum posteriorly the cerebrum is 

 dimiiautive. 



The ceiebellum is still large but it is crumpled. The pnra- 

 flocculi no longer bulge over the acoustic region, but are 

 contracted towards the lobus medius. The latter is very 

 asymmetrical. The space between the middle cerebral vessels 

 and the jugular leash, and lateral to the paraflocculus, seems to 

 have been devoid of brain-substance in the recent condition and 

 to have been occupied by venous sinuses. 



