Exterior has sculpture of narrow radiating whitish ribs that project as points from the aperture-rim of unworn shells.
Grooves between ribs are brown where uneroded.

OTHER SPECIES ALBUMS marinvert.senckenberg.science/image-browse/

Identification of British patellid limpets.
Shell-exterior cannot be relied on, and shell-interiors can be confusing. Examination, in good light under magnification, of extended pallial tentacles on living animals is essential for consistent accurate discrimination of the three rock-dwelling Patella species. Best achieved with specimen adhering to underside of supported glass-sheet in black-based container of seawater.
Some morphologically intermediate forms can only be reliably identified by sequencing DNA or allozyme study 36 Patella depressa – probable. Features intermediate between spp. Lleyn, Wales. October 2015. Leg. S. Payne. and 37 Patella depressa – probable. Features intermediate between spp. Lleyn, Wales. October 2015. Leg. S. Payne. . Intermediates result from similar environmental factors affecting different species in similar ways and are not hybrids (Sanna et al., 2011 and Sá-Pinto et al., 2007). For the purpose of recording for distribution schemes it is advisable to disregard intermediates unless DNA or allozymes can be employed, especially beyond or on the limits of known distributions. Intermediates most frequent near limit of distributions of P. depressa and P. ulyssiponensis in Isle of Wight , perhaps because conditions not optimal (Fretter and Graham, 1994).

Key identification features of typical British specimens.

Patella depressa
[1&2 in combination, not singly, diagnostic of typical specimens but excludes intermediates.]
1. Pigmented pallial tentacles are opaque chalky-white for more than half of extended-length; may have translucent tip; distinctly whiter than buff mantle-skirt from which they arise 20 Patella depressa. Length 27mm. Lleyn, Wales. September 2015. . Even when mantle-skirt retracted, pallial tentacles often clearly visible contrasting with the darker mantle 17 Patella depressa. Length 26mm. Lleyn, Wales. September 2015. .
2. Sole of foot pitch-brown 16 Patella depressa. Length 30mm. Lleyn, Wales. September 2015. to black 17 Patella depressa. Length 26mm. Lleyn, Wales. September 2015. .
3. On shell-interior, whitish projecting points of ribs have short, unglazed, chalky, pure-white central line, but reduced or lacking where projecting points of ribs eroded 08 Patella depressa. Length 30mm. Lleyn, Wales. September 2015. . [This feature recently recognised by S. Payne, and applies to all in large sample examined by IFS. Unsure yet if universal on P. depressa and exclusive of P. vulgata and P. ulyssiponensis.]

Before making records of this species further north than NW Anglesey it is advisable to familiarize oneself with specimens in areas where it is known to be frequent such as S. Devon, Channel Islands or Brittany.
Confirmation/correction can be sought by posting clear photo of pallial tentacles and foot on British Marine Molluscs Group at www.facebook.com/groups/british.marine.mollusca/

Similar species

Patella ulyssiponensis
1. Pallial tentacles have opaque pigment; white, off-white, cream or, on large specimens, yellowish or orange for about half of length; distal-half fades to translucent. Opaque basal parts often distinct from translucent mantle-skirt that they arise from so possible to confuse with P. depressa; important to use pallial tentacles in combination with foot-colour/shell-length for identification 41 Patella depressa album. COMPARISON images of pallial tentacles on P. ulyssiponensis. Shell lengths A10.6mm, B12mm, C17mm and D40mm. North Yorkshire. September 2014. .
2. Foot not pitch-brown/black; whitish when young 42 Patella depressa album. COMPARISON images of foot on young P. ulyssiponensis. Shell lengths 10.6mm, 12mm and 17mm. North Yorkshire. September 2014. becoming yellowish 43 Patella depressa album. COMPARISON images of foot on adult P. ulyssiponensis. Shell lengths 20.3mm, 24.7mm and 26,8mm. North Yorkshire. September 2014. and often orange with age 44 Patella depressa album. COMPARISON images of foot on large adult P. ulyssiponensis. Shell lengths 39.6mm, 40mm and 40.5mm. North Yorkshire. September 2014. . Beware of juveniles under 12mm length that lack gonads above sole as dark viscera may show as blackish-shadow through thin pale translucent foot 42 Patella depressa album. COMPARISON images of foot on young P. ulyssiponensis. Shell lengths 10.6mm, 12mm and 17mm. North Yorkshire. September 2014. . Green ovaries resting on interior surface of foot of mature female may show as faint greenish tinted zone along median line of foot where it is thinnest 43 Patella depressa album. COMPARISON images of foot on adult P. ulyssiponensis. Shell lengths 20.3mm, 24.7mm and 26,8mm. North Yorkshire. September 2014. .

Patella vulgata
Extremely variable species; foot colours and nearly all shell-features have overlaps with P. depressa and P. ulyssiponensis.
1. Pigment-less pallial tentacles are slender, translucent and same colour as mantle-skirt they arise from 45 Patella depressa album. Image of pallial tentacles on P. vulgata for comparison. Great Orme, Wales. .
Cautions:
Pallial tentacles may look white when arise from colourless mantle-skirt in some lighting, but no pigment and not chalky-white 46 Patella depressa album. Image of pallial tentacles on P. vulgata for comparison. Great Orme, Wales. .
Pigment-less translucency and fineness often make discernment difficult, especially when mantle skirt retracted from shell-rim and pallial tentacles viewed against shell 47 Patella depressa album. Image of pallial tentacles on P. vulgata for comparison. Menai Strait, Wales. June, 2013. ; often virtually invisible when out of water as may be retracted into mantle 48 Patella depressa album. Image of pallial tentacles on P. vulgata out of water for comparison. Menai Strait, Wales. September 2009. .
Foot colour varies, sometimes as dark as on P. depressa 47 Patella depressa album. Image of pallial tentacles on P. vulgata for comparison. Menai Strait, Wales. June, 2013. .