The species in the subclass Haplomitriidae have leaves that are 3-ranked on the stem. A closer look will reveal the presence of two rows of lateral leaves and one row of underleaves, the latter being reduced. Oil bodies, which are small, can be found within the leaf cells of these species. Although the plants within this subclass lack rhizoids they have an extensive rhizomatous network.
Species in Haplomitriidae can either be diocious or monoicous and the female reproductive structures may be either acrogynous or anacrogynous. Althought the perianth is lacking, the female protective tissue surrounding the developing sporangium may be a shoot-calyptra or a true calyptra.
The sporophyte, which is typically cylindrical in shape and robust, has a sporangial jacket that is unistratose. A closer look at the sporangial wall will reveal that each cell has a single transverse band thickening. Spore dispersal in this subclass generally occurs along 1, 2, or 4 lines of dehiscence.