Disarticulated opercular valves (treated with bleach). Different faces of right tergum 1, 3, and scutum 2, 4. On live animal, faces 1, 2 visible, usually fixed together even when operculum open. Portland Bill, Dorset. April 2012

The fatness/slenderness of the scutum outline is variable and should not be used to differentiate this species from its congener ,C. stellatus. The small size of the tergum of C. montagui is a more reliable way to separate them.