Thank you, Angela.
The registry date lozenge should translate to 6 December 1881 - Parcel 13, corresponding to W.H. Heppell's RD 374437.
The design was presumably for the shell decoration rather than the shape because there is a creamer, sugar and covered butter with the same lozenge. The only other colour I have seen these in has been a somewhat muddy purple marbled glass (which Raymond Slack describes as being the same as Sowerby's brown malachite, though not to my eyes).
Fred.