Calculus bicolor, the sole species of the genus Calculus, is a South African spider in the family Orsolobidae. Individuals are 4 mm in length, although only juveniles have been described. The abdomen is pale yellow with a broad brown patch and black markings on the sides of the spinnerets.[1] Calculus bicolor was described in 1910 by William F. Purcell, and long assigned to the Oonopidae (goblin spiders), until a 2012 study assigned Calculus to the family Orsolobidae on the basis of sensory organs that differed from those of oonopids.[2]