Plants medium-sized, green, yellowish, or with red secondary coloration; branch and shoot apices not pencil-like; cells of stem epidermis usually widened in part of stem circumference and forming a partial hyalodermis; pseudoparaphyllia triangular, broadly triangular or broad and irregular; axillary hairs with 1--4-celled distal portion, hyaline when young. Stem leaves falcate or strongly so, more rarely straight, gradually narrowed to acuminate apex from an ovate or ovate-triangular base, concave; margins distinctly denticulate in distally or proximally or both; costa ending 60--95% distally in leaf; alar cells in distinctly delimited, transversely triangular group that reaches costa or almost so, not decurrent, supra-alar cells weakly differentiated. Sexual condition dioicous.

Among the wetland species of the genus with leaves that narrow gradually to the leaf apex and are gradually curved throughout, Warnstorfia exannulata is the most frequent. In W. exannulata, the marginal lamina cells in the widest part of the stem leaves are often distinctly rectangular or distinctly widened or both. This is never as pronounced in other Warnstorfia species as it can be in W. exannulata. However, because this feature is rather variable it should be used with caution when identifying material. Warnstorfia fluitans and W. pseudostraminea differ from W. exannulata in being autoicous, usually having narrower triangular to lanceolate pseudoparaphyllia, hardly ever getting pure red colors, and having more weakly differentiated alar groups. The alar groups in W. fluitans are usually narrower, i.e., reaching less far distally in the leaf than in W. exannulata, whereas those in W. pseudostraminea often form ovate groups together with the supra-alar cells. Certain phenotypes of W. exannulata from springs, with, i.a., the inflated alar cells more or less in only one row along the leaf base, have sometimes been called var. purpurascens (Schimper) Tuomikoski & T. J. Koponen. However, all transitions between these phenotypes and those from other habitats exist, and if the different phenotypes are cultivated together the resulting plants cannot be separated from each other (based on north European material). In late snow beds or in cold springs in the higher mountains and in the Arctic, modifications with less well delimited alar cells occur.

Intermediately mineral-rich fens, often around springs or in late snow-beds, sometimes submerged in lakes; 0--4150 m; Greenland, Alta., B.C., Man., N.B., Nfld., N.W.T., Nun., N.S., Ont., Que., Sask., Yukon; Alaska, Calif., Colo., Idaho, Maine, Md., Mass., Mich., Minn, Mont., Nev., N.H., N.Y., Oreg., Pa., Utah, Wash., Wyo.; Mexico; South America; Eurasia; Africa; Australia; Pacific Islands (New Zealand).

The only difference mentioned in the literature between the most common expressions of W. exannulata and its fo. orthophyllus Mönkemeyer is in the leaf curvature. Although the type of the latter was not seen, a separate taxon cannot be recognised for straight-leafed plants.