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]