Roy sent me a photo of a similar item (also unmarked and in purple marbled glass) some time ago, though Roy described it as a sugar bowl.
The shell design of the bowl is similar to that of the Heppell RD 374437 set of 6 December 1881 (comprised of a sugar bowl, creamer, and covered butter dish) - also often found unmarked as Davidson pattern number 190 from their unregistered designs 1880-1890. I've seen the Heppell/davidson set in purple marbled glass and in white opaque glass.
There are several differences, however, between the Heppell RD 374437 sugar bowl and your piece. The Heppell sugar has a domed and ribbed oval foot, whereas your sugar has a rectangular foot with grooves running from the corners to the centre. The coils on the 'shell' bowl of your piece have numerous additional horizontal ribs, whereas the Heppell sugar has coils that are basically plain but are separated into three groups by two widely spaced cross ribs.
So far, I have only seen pieces like yours in purple marbled glass, and I haven't any other shapes in the pattern.
Sadly, on the evidence so far, I think that your shell vase (might it be a small sugar bowl?) will have to remain unattributed. Another case of an undocumented Davidson unregistered design or just perhaps opportunist design plagiarism?
Fred.