The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]