Plants in turf or mats, green to yellow-brown distally, light to dark brown proximally. Stems to 1(-3) cm, branching occasionally; oval to rounded-triangular or pentagonal in transverse section, hyalodermis absent, sclerodermis weak, cells with smaller lumens, but usually thin-walled, central strand present, strong; weakly radiculose or with red-brown tomentum; axillary hairs of 3-10 cells, all hyaline or basal 1-2 brownish. Leaves often distant or crowded, occasionally secund, 1-1.5(-2) mm, appressed-incurved when dry, spreading when moist; ligulate to lanceolate, occasionally triangular or acuminate, adaxial surface strongly keeled, deeply grooved along costa; leaf base scarcely differentiated in shape or ovate, laminal margins plane or weakly recurved in proximal ¹/2, entire, occasionally finely crenulate or weakly denticulate; apex broadly obtuse to sharply acute or occasionally acuminate; costa sometimes short-excurrent as a mucro or rarely ending a few cells below apex, adaxial outgrowths absent, adaxial cells elongate, occasionally short-rectangular to quadrate near apex, in 2-3 rows, abaxial cells elongate; transverse section oval to reniform, adaxial epidermis absent to weakly developed, adaxial stereid band absent, guide cells 2-4 in 1 layer, hydroid strand absent, abaxial stereid band strong, semicircular to ovate in section, abaxial epidermis usually distinct; basal cells differentiated in a small group at base of costa, short-rectangular, little wider than distal cells, 2-4:1, usually thick-walled; distal medial cells subquadrate, occasionally elongate transversely or longitudinally, (5-)7-9(-15) µm wide, 1(-2):1(-2), papillae either massive, multifurcating and centered over lumens or simple to 2-fid, cell walls thin to greatly thickened, superficially flat to bulging. Specialized asexual reproduction rare, by gemmae in axils of leaves. Sexual condition dioicous; perigonia and perichaetia terminal on short lateral branches, interior perichaetial leaves convolute-sheathing, ovate-acuminate, 1-1.5 mm, laminal cells shortly rhomboidal to near apex. Seta yellow-brown, 0.3-0.8 cm, twisted clockwise proximally, occasionally counterclockwise distally. Capsule yellow-brown to brown, ovoid to elliptic, 0.5-1(-1.5) mm, exothecial cells rectangular, walls thin, annulus of two rows of weakly vesiculose cells; operculum long-rostrate, 0.4-0.6(-1.8) mm, cells in straight rows; peristome teeth absen

Anoectangium is distinguishable from Molendoa, a morphologically similar genus also having very short perichaetia-bearing branches arranged laterally on the axis, by its often triangular stem section, constant lack of a ventral costal stereid band, the leaves keeled by a deep adaxial groove along the costa, and the nonglaucous color in nature. Hymenostylium has similarly though not as deeply keeled leaves with elongate cells on the adaxial surface, but has terminal perichaetia, lacks a stem central strand, and medial laminal cells are commonly longitudinally elongated. Amphidium species are similar in appearance but the sexual condition is autoicous (A. lapponicum) or papillae are simple and elliptical (A. mougeotii). Species of Zygodon may be distinguished by the simple, isolated papillae, not obscuring the lumens. Although the lack of an adaxial stereid band helps separate Anoectangium from robust specimens of Gymnostomum and Molendoa, variation in expression of this trait requires other traits for distinguishing small forms.

Excluded Species:

Gyroweisia pusilla (Kindberg) Brotherus

This entity was treated as a synonym of Gymnostomum calcareum by A. L. Andrews (1922b).

Gyroweisia reflexa (Bridel) Schimper

This is a peristomate species known for the flora area previously only from a single collection in Grey County, Owen Sound, Ontario (Moxley s.n., May 1924, CANM). The specimen is indeed that, but H. A. Crum and L. E. Anderson (1981) questioned the report, suggesting a confusion of specimens by Moxley (as reviewed by W. R. Manierre 1998). The region of the original station, now built over, has been examined by others, but no additional sites discovered. The Michigan collection (MO) reported by Manierre is Tortula obtusifolia (Schwägrichen) Mathieu, which, though similar in the ligulate leaves and rudimentary peristome, differs significantly in the crowded, short-ovate leaves, and the single stereid band and enlarged adaxial superficial cells of the costa in section. Although it would be difficult to demonstrate the absence of this species, given the circumstances it is now appropriate to consider it absent from the New World.