Rustyback Ceterach officinarum (formerly Asplenium ceterach) is the least common of the ferns found on the walls here. It is however thankfully fairly distinctive because of the fine scales which cover the underside of the leaf segments. These appear golden brown at this time of year but in the spring are a sparkling silvery mass. Under the microscope the scales are actually made of large cells which are translucent centred but coloured around their edges. The underside of the rachis appears the same green as the rest of the leaf surface but the stipe is covered in dark scales.
At this time of year the sori seems to have dropped off but the sori scar shapes are still there. Like polypody, this fern lacks an indusium.