amnion vs pileus

amnion

noun
  • The innermost membrane of the fetal membranes of reptiles, birds, and mammals; the sac in which the embryo is suspended. 

pileus

noun
  • The top of the head of a bird, from the bill to the nape. 

  • A small thin cloud attached to a cumulus cloud. 

  • The bell of a jellyfish. 

  • The cap of a mushroom. 

  • A conical felt hat worn in Ancient Rome and Greece. 

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