460 PALEONTOLOGY 



survived from the Cretaceous, now appear for the last 

 time, alongside Schizaster and others which continue to 

 the Recent period. 



Mammalia include the first antlered deer, first horned 

 rhinoceros, first ape. Ancestors of elephants suddenly 

 appear in Europe. 



13. PLIOCENE. During this period there is in the 

 northern hemisphere a gradual southward migration of 

 the marine fauna, Arctic species appearing in Britain and, 

 later, in the Mediterranean area. The horizon of any 

 normal marine bed in the Pliocene system can be deter- 

 mined by the percentage of its species which are 

 (a) extinct, (b) surviving in more northern or (c) in more 

 southern latitudes. 



Mammalia attain their greatest size. The first one-toed 

 horse appears. Mastodon is characteristic. 



