The Collaborative International Dictionary of English v.0.48:

Snake's-head \Snake's"-head`\, n. (Bot.)
   The Guinea-hen flower; -- so called in England because its
   spotted petals resemble the scales of a snake's head. --Dr.
   Prior.
   [1913 Webster]

   Snake's-head iris (Bot.), an iridaceous plant
      (Hermodactylus tuberosus) of the Mediterranean region.
      The flowers slightly resemble a serpent's open mouth.
      [1913 Webster]