Herbs short-lived perennial or annual, 15--40 cm tall. Stems caespitose or simple, suberect; sterile stems decumbent; flowering stems pilose and/or glandular pubescent all round. Basal leaves ovate, obovate-lanceolate, ovate-spatulate, or spatulate, 5--13 × 3--10 mm, both surfaces pilose, base attenuate into a petiole; cauline leaves subsessile, ovate, oblong, or narrowly ovate-oblong, 1--3(--4) × 0.3--1(--1.2) cm, both surfaces pilose or pubescent, margin densely ciliate, apex acute. Cyme terminal, spreading, lax, up to 40-flowered; bracts leaflike, ovate, 3--5 mm, both surfaces glandular pubescent. Pedicel slender, 0.5--2.5 cm, densely glandular pubescent, recurved after anthesis. Sepals oblong- or ovate-lanceolate, 5--6 mm, abaxially densely glandular pubescent, margin membranous or narrowly so. Petals shorter than or equaling sepals. Petals obovate or obovate-oblong, shorter than to 2 × as long as sepals, apex 2-lobed. Stamens shorter than petals. Ovary ovoid, ca. 2 mm. Styles 5, linear, slightly longer than ovary. Capsule cylindric, 8--10 mm, ca. 2 × as long as calyx, 10-toothed, teeth usually recurved. Seeds brown, usually 0.4--0.8 mm in diam., tuberculate. Fl. Apr--Jun, fr. May--Jul.
Forests, forest margins, mountain slopes, hilltop grasslands, fields, sandy soils, rock crevices, roadsides; 100--4300 m.
2. Cerastium glomeratum Thuillier
Herbs annual, 10--20 cm tall. (Annual herb with straggling, ascending stems up to 45 cm as per POWO) Stems simple or caespitose, densely villous, distally glandular pubescent. Proximal leaves spatulate; distal leaves obovate-elliptic, 1.5--2.5 × 0.5--1 cm, base attenuate into a short petiole, both surfaces villous, midvein prominent, margin ciliate. Inflorescence of compact, cymose clusters (glomerules); rachis densely glandular pubescent; bracts leaflike, ovate-elliptic, densely pubescent. Pedicel 1--3 mm, densely pubescent. Sepals 5, lanceolate, ca. 4 mm, abaxially densely long glandular pubescent, margin narrowly membranous, apex acute. Petals 5, white, oblong, subequaling or slightly longer than sepals, base pilose, apex 2-lobed. Stamens shorter than sepals. Styles 5. Capsule cylindric, 1.5--2 × as long as calyx, 10-toothed. Seeds brown, compressed triangular, tuberculate. Fl. Mar--Apr, fr. May--Jun.
Forest margins, mountain slope grasslands, sandy riversides; 100--3700 m.
So the main difference appears to be in cyme terminal, spreading, lax, up to 40-flowered, Sepals oblong- or ovate-lanceolate and Petals shorter than or equaling sepals. Petals obovate or obovate-oblong in Cerastium fontanum subsp. vulgare (syn. of Cerastium holostedides) compared to inflorescence of compact, cymose clusters (glomerules), Sepals 5, lanceolate and Petals 5, white, oblong, subequaling or slightly longer than sepals in Cerastium glomeratum Thuillier.