Small perennial (or Polypremum reportedly sometimes annual) mat-forming to upright herbs, terrestrial, subaquatic or aquatic. Leaves cauline, opposite, simple, sessile or subsessile, joined basally by a transverse membrane; lamina linear and minutely scabrid (Polypremum) or broadly ovate and margins more or less ciliolate (Tetrachondra), entire or rarely distantly denticulate. Flowers mostly solitary or in leafy cymes, actinomorphic, 4-merous (sometimes reported as infrequently 5-merous or very rarely with 6 sepals in Polypremum), bisexual. Calyx campanulate, persistent; sepals, narrowly ovate, with apex ± obtuse, connate at base. Corolla white, tube subrotate to campanulate, with a ring of moniliform hairs in the throat (Polypremum) or lacking a ring of hairs in the throat (Tetrachondra); petals connate at base. Stamens alternate with petals, adnate to corolla; dithecal, dorsifixed, introrse, dehiscing by longitudinal slits. Gynoecium of 2 connate carpels. Ovary semi-inferior (Polypremum) and style terminal, or superior (Tetrachondra) and style gynobasic, 2-locular (Polypremum) or appearing as 4-locular by development of a false septum (Tetrachondra); stigma slightly truncate to capitate. Ovules 1 (Tetrachondra) or 20–30 (Polypremum) per locule; placentation basal or the ovules on peltate placentae attached to septum. Fruit either a 2-valved, somewhat laterally compressed capsule (Polypremum), usually ovate in outline, nested in a persistent calyx, with 20–30 seeds per locule, initially loculicidal, later septicidal, or a schizocarp of 4 setulose nutlets (Tetrachondra) each with a single seed. Seeds angular, cuboidal (Polypremum) or ovoid to ellipsoid-trigonous (Tetrachondra), not winged, with copious endosperm.
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