microscleres of C- or S- shape. There are a number of distinct types, bearing different names derived from the basic sigma shape (meniscoid microscleres):
-chiastosigma
-clavidisc
-comma
-cyrtancistra
-diancistra
-sigmancistra
Furthermore several non-homologous (?) microscleres resemble sigmas:
-sigmaspire
-spirosigma

Alternative forms for sigma : sigmas, sigmata.