Pleurosigma :: Pleurosigma (n.) 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..
Sigma :: Sigma (n.) The Greek letter /, /, or / (English S, or s). It originally had the form of the English C..
Sigmodont :: Sigmodont (n.) Any one of a tribe (Sigmodontes) of rodents which includes all the indigenous rats and mice of America. So called from the form of the ridges of enamel on the crowns of the worn molars. Also used adjectively.