BUAIN OF THE ZEUGLODONTID^. 



641 



■the retention of elaborate visual capacities in the Miocene — an 

 utter impossibility in the offspring of the Zeuglodonts here 

 described at any epoch. 



It is of some value to appreciate how great a degree of 

 ^trigeminal specialisation is compatible with future evolution. 

 For it is evident that Prosqualodon davidi has an enlarged 

 trigeminus, even though we do not find such gross enlargement 

 of the Gasserian ganglia as in the Zeuglodonts. It has already 

 been indicated that " a certain degree of trigeminal specialisation" 

 is to be expected in the ancestors of Cetacea. It seems unques- 

 tionable that the initial widening of the Prosqualodont, as Well 

 as that of the Zeuglodont, brain and medulla oblongata is due 

 not merely to the passive recession of the sense of smell but 

 rather to the active hypertrophy of the trigeminal apparatus, 

 which in an aquatic mammal provides so much more information 



Text-figure 20. 



Lobus medi'us cerebe///. 

 (generg//y ej<pan&e&.j' 



Mec/uJ/s ob/onggpa. Trigeminus. 



Posterior view of endocranial cast of Trosqualodon davidi Flynn. 

 About J iiat. size, 



•concerning food, friends, and foes [than do the senses of smell 

 ■or sight. 



Prosqualodon davidi teaches us therefore that the evolution of the 

 Cetacean stock, while it depended to some degree upon an initial 

 trigeminal specialisation, was not effected by any sudden reliance 

 upon this sense to the neglect of other important senses, but 

 depended upon an orderly and " balanced " correlation of this 

 hypertrophy with a concurrent aggrandisement of the visual and 

 auditory senses. In this connection it is significant that the most 

 expanded portions of the fore brain in Prosqualodon davidi appear 

 to be the *' occipital " and " temporal "■■ regions ; i. e., posteriorly 

 and laterally, where one may reasonably conclude that these 

 senses were finding cerebral representation. Whether Fraas is 

 correct in believing that even the more primitive Protocetus 



