154 EXTINCT MONSTERS. 



remains have not yet been discovered, or where, for various 

 reasons, their skeletons were not preserved. In this Eocene 

 lake, where sediments went on being quietly deposited for a long 

 time, we have the most favourable conditions for the preserva- 

 tion of the different forms of life that flourished round its 

 borders. 



In the museum at Yale College are collected the spoils of 

 numerous expeditions to the West, and the many tons of bones 

 lying there are believed to represent the remains of no less than 

 two hundred individuals of the Dinocerata. So perfectly have 

 these bones been preserved by Nature that, even if the creatures 

 had been living now, the material for studying their skeletons 

 could hardly be more complete. Professor Marsh recognises 

 three distinct types in this strange group of quadrupeds, on each 

 of which a genus has been founded. The first and oldest form is 

 the Uintatherium, which takes its name from the Uinta Mountains. 

 This, as might be expected, is the most primitive or least 

 specialised form, and comes from lower strata. The most highly 

 developed or specialised form is the Tinoceras, and this is found 

 at the highest geological level or "horizon." 



Between these two extremes, and from an intermediate 

 horizon, comes the Dinoceras, 1 so that in tracing these animals 

 through the strata in which they occur the geologist finds that he 

 is following for a while the course of their evolution. Doubtless 

 there were many slight differences presented by the members of 

 this group, but at present it has not been found possible to 

 determine the number of species, although about thirty forms 

 more or less distinct have been recognised. Professor Marsh 

 says that the specimen of the skull of Dinoceras mirabile, on 

 which the whole order Dinocerata was founded, is, fortunately, 

 in a very perfect state of preservation, and that it belonged to a 

 fully adult animal. Moreover, it was embedded in so soft a 



1 The Dinoceras of Marsh is the same form as Eobasileus of Cope. Uinta- 

 therium was discovered by Leidy. 



