Grimmia decipiens looks very similar to the ubiquitous G. pulvinata, although it is much less common and mostly found in hilly districts. If you are in such an area and come across what seems to be a large, loose cushion of G. pulvinata you may have found G. decipiens.
In the field, two features suggest G. decipiens. First of all, with a good x 20 hand-lens you should be able to see that leaves have a distinctly roughened hair-point. This is down to small but distinct sharp teeth, best seen on young to mature leaves. Confirm this under a high-power microscope but be careful – G. pulvinata often also has teeth on the hairpoint, although these are more obscure and less spiny-looking. Leaf shape is another good means of homing in on potential G. decipiens in the field. As you can see from some of the images below, the leaves of G. decipiens taper gradually into the hairpoint. Among British and Irish Grimmia spp., only G. pulvinata and G. orbicularis have an elliptical leaf shape and this means the lamina contracts quite abruptly into the hairpoint.
As with the majority of Grimmia species, microscopic examination is essential. In particular, look at the base of the leaf. Well-developed G. decipiens normally has strikingly long and narrow basal cells with thickened, nodulose walls adjacent to the nerve which contrast with a few rows of short, hyaline cells near the leaf margin – there are some good images of this below.Read the Field Guide account