258 EXTINCT MONSTERS 
succulent 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. 
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,' 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 Dincoeras of Marsh is the same form as Eobasileus of Cope. Uinta- 
therium was discovered by Leidy. 
