discoid vs discoidal

discoid

adj
  • Shaped like a disc/disk. 

noun
  • A disk-shaped dental excavator designed to remove the carious dentin of a decayed tooth 

discoidal

adj
  • Having the flat, circular shape of a disc or a quoit 

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