Holdfast dwellers position themselves upside down and eat a deep pit into the core of the stipe from below. The shell becomes distorted and often coated with epizoic growths. The aperture rim strongly curves from the horizontal at the ends to fit the bowl-like base of the pit.
Much of the holdfast is cut away to expose the pit.

Concise
Part 1, SHELL FORMS: 02 Patella pellucida. Growth Stage 2 (GS2). July 2012. Littoral. North Yorkshire, England. © P. Lightfoot.
Part 2, BODY & ANATOMY: 03 Patella pellucida. Growth Stage 3 (GS3). 2009. Sublittoral, Farne Islands, Northumberland, England. © P.Lightfoot.
Part, 3 HABITS & ECOLOGY: 04 Patella pellucida. Length 4.6mm. Growth Stage 3 (GS3). September 2016. Yorkshire, England. © P.Lightfoot.