The Collaborative International Dictionary of English v.0.48:

lingula \lin"gu*la\ (l[i^][ng]"g[-u]*l[.a]), n.; pl.
   lin"gu*l[ae] (l[i^][ng]"g[-u]*l[=e]). [L., a little
   tongue.]
   1. (Anat.) A tonguelike process or part.
      [1913 Webster]

   2. (Zool.) Any one of numerous species of brachiopod shells
      belonging to the genus Lingula, and related genera. See
      Brachiopoda, and Illustration in Appendix.
      [1913 Webster]

   Lingula flags (Geol.), a group of strata in the lower
      Silurian or Cambrian system of Wales, in which some of the
      layers contain vast numbers of a species of Lingula.
      [1913 Webster]