Riella
Aquatic on mud, monoicous or dioicous with female and male plants of similar size. Multicellular gemmae often produced on abaxial surface of stem. Plants comprising a simple or sparingly furcate axis that gives rise to prominent dorsal thallose wing or rarely two wings (not in Victoria), and two lateral and two abaxial rows of much smaller leaf-like scales; lateral and abaxial leaf-like scales triangular, oblong, ligulate or filamentous; thallus cells quadrate to hexagonal, several with a single spherical rough oil body scattered throughout. Antheridia along wing margin, in wing sinuses or rarely between two wings on adaxial axis (not in Victoria). Pseudoperianth subglobose, ovoid or pyriform, sessile or stalked, with (not in Victoria) or without longitudinally oriented ribs. Capsule globose to ovoid, along the stem and each on the opposite side of the dorsal wing than the last or rarely between two wings (not in Victoria). Spores globose, hemispheric to tetrahedral, spinose with or without a faint triradiate mark on proximal face, red- or dark brown.
28 species sporadically distributed throughout the world, mostly in regions with seasonally arid climates that creates temporary pools but also in northern temperate regions with more evenly distributed rainfall such as central Europe and USA (Söderström et al. 2016).
Söderström, L., Hagborg, A., von Konrat, M., Bartholomew-Began, S., Bell, D., Briscoe, L., Brown, E., Cargill, D.C., Costa, D.P., Crandall-Stotler, B.J., Cooper, E.D., Dauphin, G., Engel, J.J., Feldberg, K., Glenny, D., Gradstein, S.R., He, X., Heinrichs, J., Hentschel, J., Ilkiu-Borges, A.L., Katagiri, T., Konstantinova, N.A., Larraín, J., Long, D.G., Nebel, M., Pócs, T., Puche, F., Reiner-Drehwald, E., Renner, M.A.M., Sass-Gyarmati, A., Schäfer-Verwimp, A., Moragues, J.S., Stotler, R.E., Sukkharak, P., Thiers, B.M., Uribe, J., Váňa, J., Villarreal, J.C., Wigginton, M., Zhang, L. & Zhu, R. (2016). World checklist of hornworts and liverworts. Phytokeys 59: 1–828.