48 JoURNAL OF COMPARATIVE NEUROLOGY. 
determine the size of the brain. These creatures began the 
struggle for existence in Eocene times with relatively large 
brains, in spite of their aquatic mode of life; and they have 
been succeeded by generations ot descendants whose latest 
progeny at the present day have a brain-equipment only 
slightly superior to their earlier Tertiary ancestors (vzde Cata- 
logue, of. cit. p. 344, ef seg.). Evenif we admit that the modern 
Manatees and Dugongs lead an eminently safe and retired life, 
which is in marked contrast to the venturesome and ‘‘open’’ life 
of the whales and porpoises, much still remains to be satisfac- 
torily explained. 
Perhaps the most striking feature of the brain of Zeuglodon 
is the extreme disproportion between the size of the enormous 
cerebellum and the diminutive cerebrum. In this respect the 
fossil brain presents a most marked contrast to that of all recent 
mammals, and especially to that of the Cetacea. This relatively 
great size of the cerebellum is not peculiar to the Archzoceti, 
but is common to other extinct mammals of large size. In my 
memoir on the brain in the Edentata’ the difficulty presented 
itself of adequately explaining a similar phenomenon in Glypto- 
don; and it must be born in mind, in even attempting to do 
this, (1) that the obtrusive greatness of the cerebellum presents 
itself only in large mammals and not in lowlier vertebrates, and 
(2) that the size of the cerebellum is not proportionate to that 
of the cerebrum. In the case of Glyptodon I four years ago at- 
tempted to explain these facts in this manner. 
The development of the neopallium in mammals opens up 
the possibility of the performance of many more complex mus- 
cular acts than are possible in the Amphibia or Reptilia; these 
acts require a co-ordinating mechanism, the size of which will 
be largely determined by the bulk of the muscular masses, the 
actions of which are to be harmonised, and the extent of the 
sensory surfaces which send into the cerebellum streams of con- 
trolling impulses. A large cerebellum is being demanded by a 
1 «The Brain in the Edentata,’’ ‘Linnean Society’s Trans.,’ 2nd series, 
Zoology, vol. 7, part 7, 1899, p. 381. 
