Source : Webster's Revised Unabridged Dictionary (1913)

Pleurosigma \Pleu`ro*sig"ma\, n. [NL. See {Pleuro-}, and
   {Sigma}.] (Bot.)
   A genus of diatoms of elongated elliptical shape, but having
   the sides slightly curved in the form of a letter S.
   {Pleurosigma angulatum} has very fine striations, and is a
   favorite object for testing the high powers of microscopes.