SOME EARLY MAMMALS 257 



of all the neck vertebrae ! 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 



