Plants small to medium-sized, in loose or dense tufts, mats or compact sods, fragile, dull green, yellowish to dark green distally, black, brown or tan proximally with prominent shining costae. Stems erect, often branched, in transverse section with hyalodermis, somewhat weak sclerodermis, central strand absent or present, rhizoids dense or few at the base, occasionally tomentose; axillary hairs long, 1-seriate, of 10-20 cells, hyaline. Stem leaves cirrhate-crispate to incurved when dry, spreading to recurved when moist, elongate-oblong to linear-lanceolate or linear-subulate, widest at or near the base; base hyaline, oblong, erect; margins plane to incurved distally, generally entire and minutely crenulate by projecting papillae, but often slightly or irregularly scalloped by indentations at points of laminal weakness, occasionally somewhat to strongly undulate, rarely with a border of elongate, clear, smooth cells in one series beyond midleaf, gradually tapering distally or more or less abruptly narrowed; apex acute or obtuse, cucullate or concave, with an apiculus, mucro or short subula; costa strong, percurrent to short-excurrent, adaxial and abaxial epidermal cells often present, often interrupted, adaxial and abaxial stereid bands and 1 layer of median guide cells present, hydroid strand occasionally present; proximal cells enlarged, laxly long-rectangular, thin-walled, hyaline, occasionally brown and rather thick-walled, smooth, abruptly differentiated from the green cells distally or gradual in transition, limit of the proximal region usually appears as a V, sometimes a U, often extending distally up each leaf margin as a short or elongated border; distal laminal cells medially and distally rounded-hexagonal, chlorophyllose, frequently obscured by numerous, dense, C-shaped papillae on both surfaces. Specialized asexual reproduction at the stem apex occasional, by deciduous or fragile propaguloid leaf tips, or by deterioration of fragile leaves along zones of laminal weakness. Sexual condition dioicous, occasionally autoicous; perigonia terminal, short-foliose to gemmate or as stalked buds in leaf axils of perichaetiate plants; perichaetia terminal, leaves not or little differentiated, or distinct and long-setaceous. Seta 1(-2) per perichaetium, yellow or reddish proximally with age, to 3 cm, erect, smooth. Capsule erect and symmetric or slightly inclined, yellow to reddish brown, darker red or brown

Trichostomum is similar to Tortella in leaf shape and margin flexion, but has distal laminal cells differentiated from the proximal cells in a line straight across the leaf base or in a low, poorly defined U shape, i.e., straight across but with some smooth, hyaline, elongated proximal cells extending up the leaf margins distal to the leaf shoulder. The proximal-cell line of differentiation usually forms a distinct V shape in Tortella. The peristomes of Trichostomum are erect, often short and frequently smooth, whereas those of Tortella are long and twisted generally 2-3 times (only slightly so in T. flavovirens) and densely spiculose in noncleistocarpous species (see R. H. Zander 1993). In much of the literature the peristomes of Tortella are described as papillose, when they are actually spiculose.

Pleurochaete does not have the V-shaped area of differentiated hyaline proximal echlorophyllose cells as with Tortella, but has a median area of gradually differentiated proximal cells and a strong border of several rows of cells contrasting with both laminal and proximal cells in being abruptly longer, thinner-walled, smooth and without chlorophyll. Pleurochaete also has perichaetia, in addition to perigonia, borne laterally on short branches on the main axis of the plant.

Species in Weissia are similar to Tortella by the incurving leaf margins (generally strongly and sharply incurved throughout the leaf length) with a tendency toward cucullation in the leaf apex. The proximal cells of some Weissia species may extend slightly up the margins, as in species of Trichostomum, and most especially, Weissia jamaicensis resembles a Tortella by a proximal region with a V shape. In the following treatment, great emphasis has been put on the cross section of the distal region of the leaf in delimiting taxa and for discussing relationships.

Excluded Species:

Tortella nitida (Lindberg) Brotherus

Tortella nitida has been ascribed to the flora area by I. M. Haring (1938) and S. Flowers (1973) among others. The specimens on which those reports were based have been suggested to be a variant of T. tortuosa by H. A. Crum and L. E. Anderson (1981), who also stated that material cited as T. nitida by Haring was in fact either T. fragilis or T. tortuosa, while material from Utah, described by Flowers, was not seen by them. A few specimens labeled T. nitida from various herbaria in North America were in fact one or the other of those two species. However, the specimens cited by Haring were variously either T. tortuosa var. fragilifolia or T. alpicola, and of the two cited for Utah by Flowers, the one available for study was T. alpicola. None of the three specimens cited by Haring was T. tortuosa or T. fragilis. Tortella nitida, a European species, has proximal cells gradually, not abruptly, differentiated from the laminal cells, a costa shining on the abaxial leaf surface with no distinctive subulate propaguloid leaf apex, and leaf cells to 10 µm wide. Its leaves are usually broadly lanceolate to almost oblong-ligulate, whereas those of all of the taxa just cited are lanceolate to linear-lanceolate. Only T. alpicola and Trichostomum tenuirostre are like it in its laminal fragility and stem central strand.

Tortella mollissima E. B. Bartram

The collection reported as Tortella mollissima by P. L. Redfearn Jr. (1969) is Trichostomum tenuirostre. R. H. Zander (1994g) has made Tortella mollissima (with a Mexican type) a synonym of Pseudosymblepharis schimperiana (Paris) H. A. Crum, a species of Mexico, Central America, and South America.