The Grimmiaceae are a family of mostly small, erect, dark‑green to black mosses. As primary colonizers predominantly found on bare rock they are markedly xerophytic, but a fair number of them also occur on rocks in and along streams or rivers. Adaptation to arid conditions has been a dominant evolutionary theme within the Grimmiaceae. For this reason its characters are prone to the convergent development of xeromorphological features. The family typically has lanceolate leaves with small, thick‑walled cells, poorly differentiated alar cells and a haplolepidous peristome. Peristome structure is extremely variable in Grimmiaceae. There are two different peristome morphologies, but the different types are linked by a continuum of intermediate forms. At one extreme are genera, such as Schistidium, having peristomes with narrowly triangular teeth and no basal membrane. At the other extreme are genera such as Ptychomitrium and Racomitrium, with linear teeth and a basal membrane. The Schistidium type peristome is structurally close to that of Dicranum, while peristomes of the Ptychomitrium or Racomitrium type can be found in the Pottiaceae. The Grimmiaceae have a host of morphologically distinct characters seemly ideal for phylogenetic analysis. Unfortunately their character states are often randomly and apparently haphazardly distributed both among and sometimes within genera.