pleurosigma

from The Century Dictionary.

• noun A genus of Diatomaceæ, containing species in which the valves show, with a good microscope, a series of lines, capable, under high powers and a favorable light, of resolution into dots, and therefore furnishing excellent tests for the power of a microscope.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.