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It looks like P.aspera but I've studied Patella species for a long time now and there are always individuals that I find difficult to identify with total certainty without removing them from the rock.
In general the lower down the shore you go the greater the frequency of P.aspera as they favour areas of permanent pools and run-off channels. P.aspera usually has the shell apex in the front half of the shell (as does P.depresa) while P.vulgata has a central apex. In general, P.aspera has many fine shell ribs while there are fewer on P.vulgata. Also, it appears that P. aspera is more readily colonised by algae; the moving islands of weed in a rock-pool are usually P.aspera.
Having said all that there are many exceptions and often older P.aspera are heavily eroded and clear patterns of ribs are difficult to pick out. At this point its time to gently detach the animal from the rock (using force is fatal) and look for
White interior shell
White mantle tentacles of two sizes
By the way, P.aspera has been renamed P.ulyssiponensis.
Hope that helps
Thank you for the mini lesson. :) I did think about trying to remove it, but didnt want to hurt it for the sake of an assignment!Ill have to get someone more experienced to come with me next time and show me how to do it without hurting them.
Lat/Lng: 50.1257, -5.4826
OS grid ref: SW511308