dugong vs sirenian

dugong

noun
  • A plant-eating aquatic marine mammal, of the genus Dugong, found in tropical regions. 

sirenian

noun
  • Any of a group of aquatic, herbivorous mammals, of the order Sirenia, including the manatees and dugong. 

How often have the words dugong and sirenian occurred in a corpus of books? (source: Google Ngram Viewer )