SOME AMERICAN MONSTERS. 
153 
In fact, it was a decidedly reptilian kind of brain. Perhaps it 
may seem hardly credible, but so small was the brain of Dino- 
ceras mirabile, that it could have been pulled through the 
apertures (neural canals) of all the neck vertebras ! In certain 
marsupials of the present day we find an approach to this kind 
of brain. It seems to be an established fact, according to 
Professor Marsh, that all the Eocene or earlier Tertiary mammals 
had small brains. His researches among fossil mammals have 
led him to the important conclusion that, as time went on, the 
brains of mammals grew larger ; and thus he has been able to 
establish his law of brain-growth during the Tertiary period, a 
law which appears to be plainly recorded in the fossil skulls of 
succeeding races of ancient mammals. The importance of a 
discovery such as this cannot fail to strike the imagination of 
even the most unlearned in geology as being singularly suggestive 
and instructive. It is not difficult to picture these dull, heavy, 
slow-moving creatures haunting the forests and palm jungles 
around the margin of the great Eocene lake, into the waters of 
which their carcases from time to time found their way — perhaps 
swept down by floods. No footprints have been discovered 
as yet. 
The Dinocerata were very abundant for a long time during the 
middle of the Eocene period. The position of their remains 
suggests that they lived together in herds, as cattle do now, and 
they probably found an abundance of food in the shape of succu- 
lent vegetation round the great lake. Geological evidence points 
to their sudden extinction before the close of the Eocene period ; 
but it is difficult to understand this. Professor Marsh thinks that 
from their sluggish nature they were incapable of adapting them- 
selves with sufficient rapidity and readiness to new conditions, 
such as may have been brought about by geographical changes. 
It must be admitted, however, that the geological record in this 
region does not give evidence of any sudden change. Possibly 
they may only have migrated to some other region, where their 
