Polytrichum spp.

Common to many Polytrichid species, a row of peristome teeth are fixed to an epiphragm.  The peristome teeth are made of entire cells and do not move hygroscopically.  Whereas, Bryidae species have peristome teeth that are composed of cell fragments and move in response to moisture levels.

Atrichum undulatum

The epiphragm is an extension of the columella.  A closer look reveals the peristome teeth have smooth edges, whereas in Bryidae the edges are jagged because of the cell fragments.