Contrary to P. vulgata, average size of adult P. depressa decreases up shore (Branch, 1981a) as unable to produce very large, thick, high-domed shells, like those of some P. vulgata, to resist dessication on drained rock on upper shore, though some intermediate forms show a tendency towards a high profile.

Frequent features of typical P. depressa in Britain.
1: unglazed, chalky-white linear-streak within whitish ray where rib point not eroded away.
2: transparent mantle-skirt-layer reveals coloured rays stretching from aperture-rim to opaque-whitish pallial-groove-band [but can occur in P. vulgata and young P. ulyssiponensis].
3: amphora-shaped patch within scar of pedal retractor muscle has combination of white, grey and yellow-cream [grey in this area is often cited as evidence of P. vulgata, but is neither exclusive to, nor ubiquitous on, it.]
Frequent features of typical P. vulgata in Britain.
4: steep shell-profile, H/L 67%. [P. depressa typically c.30%.]
5: specimen found in barnacle zone on upper shore; coating of Chthamalus montagui, Semibalanus balanoides and Austrominius modesta reflects this.
For further intermediate features of this specimen see image 37 Patella depressa – probable. Features intermediate between spp. Lleyn, Wales. October 2015. Leg. S. Payne.

Forms occur that are morphologically intermediate between species-descriptions of rock-grazing Patella which can only be positively identified by sequencing DNA or allozyme study. 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). Steep shell-profile in limpets is considered to be a result of prolonged muscle-contraction to clamp down the shell to avoid desiccation in drained positions exposed to the air for long periods (Fretter & Graham, 1962) . So the profile of the illustrated specimen does not preclude it from being P. depressa if it is one that has managed to survive in conditions more usual for P. vulgata. Intermediates are more frequent near eastern limit of distribution in Isle of Wight, perhaps because conditions are not optimal (Fretter and Graham, 1994). 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. Though the illustrated specimen is probably P. depressa it was not entered into a distribution scheme because of the uncertainty, especially as it would extend the known northern limit within Cardigan Bay.

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