mother-of-pearl vs pen shell

mother-of-pearl

noun

  • The hard pearly inner layer of certain mollusk shells; nacre. 

  • A butterfly of the genus Salamis. 

adj

  • Made from or looking like mother-of-pearl; iridescent or pearly. 

pen shell

noun

  • Any of the Pinnidae, a family of large saltwater clams. 

How often have the words mother-of-pearl and pen shell occurred in a corpus of books? (source: Google Ngram Viewer )