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Discussion:
The genus Barbula has lanceolate to lingulate leaves that are often stoutly mucronate, strongly recurved leaf margins, two well-developed costal stereid bands, and long-cylindrical capsules with filamentous, spirally twisted peristome teeth. It is very similar to Didymodon, and indeed the two are sometimes considered synonymous (Smith 1978, Nyholm 1990, Frey & Kürschner 1991, Abramov & Volkova 1998). Saito (1975) distinguished the Japanese members of these genera by axillary hair features and the shape of the surface costal cells on the upper, ventral (adaxial) leaf surface: Barbula all axillary hair cells hyaline, costal cells short-rectangular to elongate; Didymodon basal cells of axillary hairs brown, costal cells quadrate. In Central America the axillary hair character consistently distinguishes the genera, but the shape of the superficial, upper, abaxial costal cells various within the genera. There are three other features that tend to distinguish the two genera. In Barbula the basal and upper leaf cells are usually strongly differentiated, the leaf papillae usually multiplex, and the peristome is usually long and distinctly twisted. In Didymodon the basal and upper leaf cells are usually weakly differentiated, leaf papillae are often absent to simple, and the peristomes are usually short and weakly twisted.
Pseudocrossidium is similar to Barbula in leaf shape, in having revolute margins and in having entirely hyaline axillary hairs. It differs from Barbula in having only a single (dorsal) costal stereid band and in having more tightly revolute leaf margins in which the interior marginal cells are enlarged and thin-walled.
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