Plants in loose to dense tufts, dull, dark or yellowish green sometimes brownish. Stems erect, (0.5-)1-2(-3) cm, simple or forked, slightly to densely radiculose basally, the rhizoids reddish near the stem, often hyaline distally, smooth or roughened, arising from the stems and the base of branches. Leaves crispate when dry, curled-contorted when wet, the limb erect-flexuose from an erect base, ovate- or oblong- or linear-lanceolate, ovate and concave basally, limb distally narrow, keeled to tubulose, gradually acuminate to a fine point; apex acute to obtuse, not deciduous; margins at the base erect and entire, recurved beyond the base or occasionally throughout, nearly entire or evenly to coarsely serrulate in the distal ¹/4-¹/3 or just at the apex; margins (1-)2(-3)-stratose, occasionally 2-stratose along costa or scattered; costa single, usually ending before the apex, proximally smooth or sometimes toothed on abaxial surface especially in apical third by projecting cell ends; 7.5-10 µm, adaxial epidermal layer absent, adaxial stereid band poorly developed, guide cells present in 1 row, abaxial stereid band strong, abaxial epidermal layer poorly differentiated from abaxial stereids or absent; basal cells elongate, rectangular, mostly to 5:1, alar cells not or little differentiated; medial and distal laminal cells short, rounded-quadrate to irregular, or short-rectangular, mostly 2:1, longer and broader along costa, occasionally oblate to triangular, smooth or sometimes spinose and bulging mammillose (strongly mammillose-papillose), nonpitted. Specialized asexual reproduction absent. Sexual condition autoicous or cladautoicous; perigonia sessile or stalked, perigonial bracts (2-)4-6(-7) mm, ovate-lanceolate, concave, smooth, acute to rounded-obtuse, occasionally with recurved tips; perichaetial bracts similar to vegetative leaves or larger. Seta 2-15 mm, smooth, yellow, tending to reddish brown with age, occasionally curved-cygneous when wet, straight when dry [C. gracilescens]. Capsule erect or inclined, oblong, ovoid, cylindric, straight or often arcuate, occasionally zygomorphic, smooth or longitudinally broadly or narrowly striate when moist, becoming more deeply furrowed to sulcate when dry, with or without weakly differentiated to distinct struma; annulus variable, absent, rudimentary, small and adherent or large, 1-3 rows of deciduous, revoluble cells; operculum obliquel Cynodontium, like Dicranodontium, has a stem central strand with a central cylinder of enlarged cells, a sclerodermis of small, thick-walled cells in 1-2 rows, and lacks a hyalodermis. It is a genus of damp or moist ledges and rock crevices of boulders and vertical cliffs and boulders, soil over rock, shaded, sheltered or occasionally exposed stations, tundra hummocks, near streams in forest and alpine areas. The present treatment is primarily based on the descriptions and discussions in various manuals. B. H. Allen (2005) gave an instructive synopsis of the genus.

The ubiquitous Ceratodon purpureus (Ditrichaceae) may resemble Cynodontium species, especially C. jenneri, when the leaves are tipped with irregular teeth, also in the strongly recurved laminal margins, which are unistratose and recurved on both sides nearly to the apex. The capsule is likewise fluted when dry. Distinguishing sporophytic characters of Ceratodon purpureus are the red-purple seta, the strumose dark-colored capsule, which is inclined horizontally at the juncture of seta and urn, and the short-conic operculum, not longly and obliquely rostrate as in Cynodontium species, which have yellow setae and capsules. The mammillose-papillose, hornlike papillae of some Cynodontium species should not be confused with those of genera in the large family Pottiaceae.

Cynodontium has long been included in Oncophorus in earlier manuals; that genus includes species with leaves abruptly narrowed from a flaring, broadly sheathing base to a wide-spreading subula, and capsules not regularly striate-furrowed when dry and old, but only weakly furrowed. Cynodontium was once commonly separated from the genus Dichodontium by the autoicous condition, but one of the two species of the latter (D. olympicum) also is autoicous. The capsules of Cynodontium are regularly striate and the leaves linear-lanceolate, whereas the capsules of Dichodontium are smooth to irregularly furrowed and the leaves are shorter and generally broadly lanceolate.

Leaves of Cynodontium often have both broadly rounded and broadly to narrowly acute apices on leaves from the same stem. This includes the perigonial buds where, even in the case of perigonia with only two bracts, one will be broad and the other narrow, introducing uncertainty in using perigonial leaf shape to distinguish species. Some sterile specimens