Monographs Details: Callicostella
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants small to medium-sized, in pale-green to golden, usually thin, extensive, flat mats. Stems creeping, scarcely to freely but irregularly branched, complanate-foliate; in cross-section without a hyalodermis, with small thick-walled cells surrounding larger thin-walled cells, central strand none; rhizoids in tufts on stems, smooth to finely roughened; pseudoparaphyllia none; axillary hairs 2-celled, with a short brown basal cell and an elongate hyaline distal cell. Leaves often contorted when dry, densely to laxly foliate, lateral and dorsal leaves differentiated from ventral leaves, ventral leaves (against substrate) often differentiated in shape and areolation, usually more acuminate and with laxer, larger cells, lateral and dorsal leaves mostly oblong to broadly oblong to oblong-ovate, gradually acuminate or more often broadly rounded to truncate and apiculate, ± asymmetric; margins not or obscurely bordered, serrulate to coarsely serrate above, rarely subentire to crenulate, ± entire below, plane; costa double, strong, often ending near leaf apex with apices connivent, often toothed, especially at apex; cells short, mostly 1-2:1, typically unipapillose with papillae over the lumina, sometimes smooth, thin- to thick-walled, porose or not, usually somewhat longer toward the insertion; alar cells not differentiated. Asexual propagula rare, when present, in leaf axils, uniseriate, brownish, papillose. Autoicous, synoicous, or less often dioicous. Peri-chaetia small, inconspicuous, on primary stems; leaves few, small, ovate-triangular to linear-lanceolate; margins serrate (but less so than in vegetative leaves) to ± entire, plane; costa double, ending ca. 3/4 the leaf length, projecting at apex; cells mostly elongate, smooth. Setae elongate, short to very long, smooth throughout to roughened above, reddish, flexuose; capsules mostly inclined to pendent, rarely erect, ovoid to cylindric, usually with a well-defined neck; exothecial cells subquadrate to short-rectangular, mostly collenchymatous; annulus not differentiated; operculum conic-rostrate, the basal cells often differentiated; columella narrowly cylindric, extending to midurn or somewhat beyond; peristome double, attached at the mouth, exostome teeth reddish, narrowly triangular, narrowly bordered, not shouldered, on the front surface with a pale median furrow with a zig-zag line down it, the plates cross-striolate below, sometimes with overlying papillae, coarsely papillose above, somewhat trabeculate at back, usually finely papillose; endostome with a medium-high basal membrane, segments papillose and/or spiculose, keeled, perforate or not, with baffle-like crosswalls, cilia none. Spores spherical, finely papillose or sometimes appearing smooth, small to medium-sized. Calyptrae mitrate, covering the operculum and apex of urn, lobed at base, naked or with a few hairs, often roughened above.
Discussion:Callicostella (Müll. Hal.) Mitt., J. Proc. Linn. Soc., Bot. Suppl. 1: 136. 1859, nom. cons.; Hookeria sect. Callicostella Müll. Hal., Syn. Muse. Frond. 2: 216. 1851; Hookeria subgen. Callicostella (Müll. Hal.) Hampe, Verh. Zool.-Bot. Ges. Wien 21: 392. 1871.
Schizomitrium Bruch & Schimp. in Bruch, Schimp. & W. Gümbel, Bryol. Eur. 5(fasc. 46-47, Hookeria): 59. 1851, nom. rej.
Discussion. Callicostella is the on-again, off-again name for what has sometimes been called Schizomitrium (cf. Crosby, 1975). The name is now conserved and stabilized. It is characterized by lack of a differentiated hyalodermis, mostly oblong, unbordered leaves with broadly rounded apices that are often abruptly acute to acuminate and apiculate, mostly strongly serrate leaf margins, more or less isodiaiuetric upper leaf cells that are mostly unipapillose but occasionally smooth, and furrowed exostome teeth.
Callicostella may be confused with some of the various generic segregates of Hookeriopsis s.l. Its leaf margins are most like those of Thamniopsis, but in that genus a hyalodermis is differentiated and the leaf cells are usually elongate. From Brymela and Trachyxiphium it differs in having short leaf cells and papillae over cell lumina rather than as projecting upper angles. Hookeriopsis s.str. differs in its short costa and elongate laminal cells.
Callicostella is very closely related to Pilotrichidium, with a hyalodermis and very thick-walled, porose leaf cells, and to Diploneuron, which differs in its distinctive costal development which is very strong with the two forks confluent at the apex.