Diagnoses d’Espèces et de Variétés nouvelles de Mousses 8: 5 (1910).

TYPE. — New Caledonia. Mt Koghis, troncs d’arbre, Franc s.n. (lecto-, designated here fide Guo in Sched. [2007]: 1909, Franc s.n. PC [PC 0083685]!; isolecto-, PC [PC 0083686!, PC 0083687]).

DISTRIBUTION IN NEW CALEDONIA. — This species is only known from the type collection and a few old specimens, without locality indicated. Thus, its distribution and ecology remain unknown.

TOTAL RANGE. — Endemic to New Caledonia (South Province).

SELECTED SPECIMEN. — New Caledonia, 1913, Franc s.n., PC0721080 (Herbarium I. Thériot) (PC).

Unknown.

Medium, young upper parts light green, main parts usually light more or less red tinged, at least the costae, creeping stems densely branched.

Medium, 10-20 mm long, 1-1.5 mm wide, straight, simple or furcate, when dry curly, with leaves erect to patent, individually twisted, strongly carinate, the apices circinate to coiled and exposed by the margin, when moist patent and sinuous with basal and middle parts recurved and apices incurved. When mounted, leaf apices remain incurved and carinate and hard to flatten.

Medium, 1.3-1.7 mm long, 0.3-0.4 mm wide, lanceolate to ligulate above oblong basal parts, apices obtuse to shortly acute or apiculate, upper parts obscure, transitional parts very short, basal parts differentiated (1/2-)1/3 the whole leaf length, costae thin, ending in apices or short excurrent in mucrones or apiculi.

Small, 7-8 µm wide, single-layered thorough, in conspicuous longitudinal rows, quadrate with rounded lumina, thick walls, not bulging, strongly papillose, marginal cells undifferentiated, transitional cells poorly differentiated, lower cells smooth thorough, rectangular elongated, 20-35 µm long, 8-10 µm wide, thick walled, lumina narrow, sinuous.

Conspicuous, sheathing the setae base, inner perichaetial leaves hyaline, oblong-ligulate to sub-triangular, apices obtuse, costae ending in the apices.

Plicate, naked.

Short, 2-2.5 mm long, vaginulae naked.

2 mm long, elliptic, smooth, rims slightly plicate, brown, erect.

Single.

In the diagnosis, Thériot (1910b) describes cells smooth thorough (omnibus laevibus, hence the name laevigatum), while, in the type, the upper cells have numerous small papillae.

According to Thériot, Macromitrium laevigatum is close to M. leratii but smaller in all parts. But, in M. laevigatum, the wide acute apices, single-layered, of the relatively shorter branch leaves are distinctive, especially from the smallest forms of M. leratii, whose branch leaves are long acuminate and locally bistratose at apices.

Macromitrium salakanum and M. semperi have similar appearance by the shape and arrangement of branch leaves as well in dry as in wet conditions, but both are larger plants and, in addition, the first have hairy calyptrae and the second aristate perichaetial leaves.