decemvirate vs octovirate

decemvirate

noun
  • A group of ten people, especially (politics) a council of ten men sharing office or power and particularly (historical) the groups of ten men who reformed and codified Roman law circa 450 BCE. 

octovirate

noun
  • A group of eight people, especially (politics) a council of eight men who share office or rule. 

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