The short, thick-walled mid-leaf cells, stout costa and wet habitat will normally suffice to place this moss in Hygroamblystegium. It is often rather small and sometimes the lamina has been eroded away by e.g. water currents, leaving only the costa sticking out from the stem like a dark hair. H. tenax has a stem leaf that tapers to a narrow, sharp tip and a messy, much-branched habit.
It sometimes gets confused with H. fluviatile but it normally avoids the base-poor, fast-flowing watercourses favoured by that plant. Cratoneuron filicinum also lives on rocks in base-rich streams and has a very stout costa and short leaf cells. It does however have stem paraphyllia and obvious, inflated and often orange alar cells which form decurrent auricles, something lacked by Hygroamblystegium species.Read the Field Guide account