Left: shell exterior.
Right: shell interior.
At maturity, thicker, more opaque, adult growth (1) replaces shell loss at the anterior, which consequently rises to change the low juvenile shell into a dome with height about about 35% of length when 10mm long. The profile is smoothly rounded, except for break in slope (2) on the anterior where new adult growth joins the juvenile growth. Growth is faster at the posterior so the skirt of adult growth is wider there (3) than at the steeper anterior. Growth lines on the adult skirt , though still slanting down towards the anterior, are now continuous around the whole shell, and are not cut off at the anterior to form a horseshoe like juvenile growth lines.
4: white line at the same angle as the indicated juvenile growth line which terminates against the adult skirt.
5: juvenile horseshoe-shape growth line.
The aperture outline is a broad oval (sometimes almost circular), and its rim is on a single plane 6 conforming to the flat surface of a Laminaria frond. The blue rays persist on the juvenile growth, and may, as on this one, extend feebly, or as faint shadowy grey, onto new adult growth. Occasionally, as on this, the thicker adult growth may develop slight radial ribs.

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.