Monographs Details: Phormidium incrustatum var. rigidum
Authority: Gardner, Nathaniel L. 1927. New Myxophyceae from Porto Rico. Mem. New York Bot. Gard. 7: 1-144. pl. 1-23.
Description:Species Description - Filaments considerably tortuous, rigid; trichome 4.8-5.2 µ diam., not constricted; cells up to 2 times as long as broad; end cell rounded and end wall very slightly thickened, color very pale purplish-olive; sheath relatively thick, smooth, firm, hyaline to dark brown at maturity.

Distribution and Ecology - Growing on bark of trees, Coamo Springs, no. 300 e, type; and on the Avail of a church, Sabana Grande, no. 952; on rocks near Coamo Springs, no. 358 a.

Discussion:The variety rigidum differs from the species in having longer cells and a firm, rigid, brown sheath.