456 THE TRIASSIC PERIOD 



Mammalia. — Still another great advance in the progress of 

 life is registered in the first appearance of the Mammals, which 

 occurred in the Trias. Mammals are the most highly organized 

 forms of animals ; but these, their earliest known representatives, 

 were very small and very primitive, giving little promise of being 

 the future conquerors of the world, as they were tiny creatures 

 which, in a measure, represent the transition from lower verte- 

 brates upward. Two American genera, Dromatherium and Micro- 

 conodon, and one European genus, Microles/es, have been recovered. 



