Opercular valves (treated with bleach). Different faces of right tergum 1, 3 and scutum 2, 4. Faces1, 2 visible on live animal. Tergum fixed to scutum (in this case,even after bleaching).
Portland Bill, Dorset. April 2012.
The width: length ratio of the scutum outline is variable and should not be used to differentiate this species from its congener ,C. montagui. The larger size of the tergum, (length 40% or more of operculum) of C. stellatus is a more reliable way to separate them.