3. Callicostella (Müller Hal.) Mitten, J. Proc. Linn. Soc., Bot., Suppl. 2. 1859. • [Latin callum, hardened or thick, costa, rib, and -ella, diminutive, alluding to strong costae].
Hookeria sect. Callicostella Müller Hal., Syn. Musc. Frond. 2: 216. 1851; Hookeria subg. Callicostella (Müller Hal.) Hampe; Schizomitrium Bruch & Schimper, name rejected
Plants small to medium-sized, in flattened mats, sordid pale green, yellowish, brownish, sometimes bluish, dull. Stems regularly or irregularly pinnate; hyalodermis absent. Leaves ± complanate, shriveled, often crisped when dry, ovate-oblong to lingulate, somewhat asymmetric, ventral leaves often more acuminate, dorsal and lateral leaves broadly oblong to oblong-ovate; margins plane, entire in basal 1/4, crenulate, serrulate, or strongly toothed in apical 1/4; apex rounded-truncate, short-apiculate; costa ending in apex, subpercurrent, divergent from base, parallel or somewhat convergent distally, distal abaxial surface toothed; laminal cells isodiametric or slightly longer than wide, irregularly hexagonal to quadrate, 1-papillose on both surfaces; marginal cells forming obscure border or not forming border. Specialized asexual reproduction unknown. Sexual condition synoicous and autoicous, infrequently dioicous; perichaetial leaf apex gradually acuminate to acute. Seta red to reddish yellow, moderately to strongly papillose-scabrous. Capsule inclined, horizontal, or pendulous, ovoid or oblong-cylindric; annulus absent; operculum abruptly short-rostrate or subulate. Calyptra mitrate-campanulate, base narrowed and shortly or deeply lobed, smooth or sometimes split on one side from base, somewhat scabrous or with few hairs distally.
Species ca. 95 (1 in the flora): se United States, Mexico, West Indies, Central America, South America, Eurasia, Africa, Pacific Islands.
Callicostella is characterized by broad and abruptly acuminate leaves with no border. Other distinctive features include the long, tough double costa and the small, isodiametric, firm-walled, 1-papillose laminal cells. The distal leaf margins are strongly serrate, with teeth often 2-fid. Callicostella may also be distinguished from closely related genera by its stem epidermis composed of small, thick-walled cells.