This globally imperiled species is an early spring annual that is restricted to wet cliffs and seeps on limestone in c. Ky., c. Tenn. and n. Ala. (D. Estes, pers. comm.). Its numbers can vary much from year to year, and several small populations in Ky. appear to have disappeared during the past 30 years. The species has often been treated in Stellaria, but it also appears similar to Sagina and Minuartia (FNA 5). In the latest revision based largely on analysis of DNA (Schilling et al. 2022), Sabulina is placed in Sagineae, while the new genus Geocarpon is established in Sclerantheae to include former Arenaria cumberlanensis and A. glabra. However, morphological keys to tribes and genera remain somewhat complex (W). S. fontinalis differs from other species in Sabulina as follows (FMA 5, W): flowers solitary in leaf axils (versus usually 5-50 in terminal crymbose cymes); petals absent (versus presenr). Compared to patula and diffusa, fontinalis tends to have larger leaves: on average, ca. 10-30 x 0.8-4 mm (versus 2-20 x 0.5-1.8 mm), narrowly oblanceolate to linear-spatulate (versus linear), glabrous (versus glabrous to stipitate-glandular) and somewhat fleshy.