Macromitrium is a sizable genus, about 350–400 species, of medium- to large-sized pleurocarpous mosses. The last treatment of the genus that included Central America was Grout (1946), which attributed 24 species to the region. The genus is often found on upper tree branches in the forest canopy, but it also commonly occurs on tree trunks and can be found on rocks and soil in open, drier habitats. Most Macromitrium species have elimbate leaves with short upper leaf cells and elongate to linear, tuberculate basal leaf cells, mitrate calyptrae and short, truncate exostome teeth that are fused for most of their lengths. However, the genus is morphologically complex, and there are Macromitrium species with various combinations of limbate leaves, long upper leaf cells, short basal leaf cells, non-tuberculate basal cells, cucullate calyptrae, and long, narrowly triangular, non-fused exostome teeth. As a result the genus is difficult to characterize or cleanly separate from a number of segregate genera (e.g., Groutiella, Macrocoma, Cardotiella) recognized for groups of species with short basal leaf cells in combination with some other distinctive feature. Groutiella has a basal leaf limbidium and so appears well differentiated from Macromitrium, but there are many limbate Macromitrium species. Macrocoma differs in having slender plants with imbricate leaves. Cardotiella has strongly tuberculate cells on well developed leaf decurrencies, however, these tuberculate cells seem to be only a remarkable amplification of the basal marginal teeth that characterizes an entire subgroup in Macromitrium.

Vitt (1994) recognized three distinctive groups for the ten Macromitrium species in Mexico. The 31 Central American Macromitrium species are more diversified and variable than those of Mexico. As would be expected from such a species-rich genus, Macromitrium has many distinctive characters. However, it is difficult to subdivide the genus because its character states occur in reticulating combinations making it impossible to find any significant correlation of characters. The 31 Macromitrium species in Central America are arranged below into 7 informal groups bringing together species that share at least one of seven distinctive features. The groups are intended to help in the handling of this large genus and may not represent monophyletic units.

Group 1. Macromitrium leprieurii and M dubium.

Group 2. Macromitrium flavopilosum, M standleyi, M. subcirrosum, M. trichophyllum, and M. ulophyllum.

Group 3. Macromitrium carionis, M. contextum, M. fragilicuspis, M. guatemalense, M. oblongum, and M. podocarpi.

Group 4. Macromitrium microstomum and M. richardii.

Group 5. Macromitrium aureum, M. cirrosum, M. frustratum, M. greenmanii, M. longifolium, M. parvirete, M. picobonitum, and M. sharpii.

Group 6. Macromitrium crosbyorum, M. echinatum, M. fulgescens, M. fuscoaureum, M. mcphersonii, and M. scoparium.

Group 7. Macromitrium punctatum and M. sejunctum.

Table 1. Distinguishing features of the Macromitrium subgroups. Character states: plant size, s small, m medium, l large; leaf border, + present, ‑ absent; upper leaf cells, lin linear, e elongate, iso isodiametric; basal leaf cells, l linear, s short rectangular; swollen teeth on basal margin, + present, ‑ absent; calyptra, m mitrate, c cucullate; capsules, p puckered, x smooth or furrowed.

Groups                         1          2          3          4          5          6          7

plant size                      l           l           l,m,ss    m,s       l,m        m         m

leaf border                    ‑           ‑           ‑           ‑           ‑           +          ‑

upper leaf cells  lin         e          iso        iso        iso        iso,e     iso

basal leaf cells               l           l           l           l           l           l           s

basal margin                 ‑           ‑           +          ‑           ‑           ‑           ‑

calyptra                        c          m         m         m         m         m         m

capsules                       x          x          x          p          x          x          x