SOME EARLY MAMMALS 267 
of all the neck vertebre! 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 
Fig. 96.—Cast of brain-cavity of Dinoceras mirabile. (After Marsh.) 
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 
S 
