Description from Flora of China
Subshrubs with slender branches arising from usually short, sometimes stout, woody stems. Leaves entire or 1-3-serrulate. Verticillasters in dense capitula or spikes. Flowers pedicellate. Calyx tubular-campanulate to narrowly campanulate, 10-13-veined, 2-lipped, throat white hairy annulate; upper lip spreading or straight, 3-toothed, teeth triangular to lanceolate; lower lip 2-toothed, teeth subulate. Corolla tube included or exserted, 2-lipped; upper lip straight, emarginate; lower lip spreading, 3-lobed, lobes subequal or middle lobe longer. Stamens 4, separate; anterior 2 longer, exserted or included; anther cells 2, parallel or divaricate. Style apex 2-cleft; lobes subulate, equal or subequal. Nutlets ovoid or oblong, smooth.
Some botanists recognize 300-400 species in Africa, Europe, and temperate Asia; others have suggested that many of these species should be treated as infraspecific taxa of Thymus serpyllum Linnaeus. China has at least 11 species, mainly N of the Huang He. Some botanists (e.g., Kamelin) have recorded several additional species from China: T. dahuricus Sergievskaya, T. gobicus Tscherneva, and T. michaelis Kamelin & Budantzev (syn. T. mongolicus Klokov, 1954, non (Ronniger) Ronniger), all fairly widespread, plus T. petraeus Sergievskaya, (Xinjiang) and T. turczaninovii Sergievskaya, (Nei Mongol). We have not examined material of these species.
Klokov (Fl. U.S.S.R. 21: 471. 1954) reported that fertile hybrids are common in areas of overlap between some species.