axle tooth vs premolar

axle tooth

noun
  • A molar tooth; molar. 

premolar

noun
  • A tooth situated in front of the molar teeth; especially a tooth in humans with two cusps which is between the canines and the molars (Latin: singular dens premolaris, plural dentes premolares) 

adj
  • Before a molar tooth, either in physical position or in its time of development; deciduous. 

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