A genus of fossil algae. Girvanella is characterized by flexuous, tubular filaments of uniform diameter, composed of thick, calcareous walls (see illus.). External diameters average between 10 and 30 micrometers, although specimens less than 10 μm and up to about 100 μm have been identified as Girvanella. Filaments may occur free (unattached), but usually occur in groups, twisted together to form nodules and encrusting masses on various objects. The genus is intergrown with encrusting foraminifers in some Paleozoic limestones.