Hi David,

Maybe you are familiar with Poppe & Goto's work : European Seashells,
vol 1 ? If not, here's the section on the Margarites groenlandicus
(hopefully not to add to your confusion):

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Margarites groenlandicus (Gmelin, 1791)
A circumboral species. In Europe it lives from northern Scandinavia
south to Scotland...
5 to 10 mm
... colour goes from green to pink and orange. In Europe the species has
heavy spiral ridges all over the shell. Specimens from Greenland are
usually smooth. Flattened shells with a closely set spiral sculpture are
called form undulata Sowerby, 1838.

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Maybe this will help you further on. Looking at the photo in Poppe &
Goto, added to their description and your description, I'm pretty sure
you will have a groenlandicus.

Best regards,

Jan Haspeslagh
Belgium

David Kirsh wrote:
>
> Dear all,
>
> I just received a couple of Margarites from Iceland in trade. I see
> that identification is not entirely straight forward.
>
> One is 7.5mm in diameter and has seven spiral ridges on the top of the
> body whorl ( on magnification, the ridges look like waves of fudge)
> and a dozen low smooth spirals on the base. My trade-mate calls it M.
> groenlandicus.
>
> However, in Abbott's Am. Seashells (2nd ed.), groenlandicus Gmelin is
> smooth on the base and has "about a dozen spiral lirations" on the top
> (an almost entirely smooth form is called umbilicalis Broderip and
> Sowerby) . Abbott's Seashells (1991, covering northern North Atlantic
> and northeastern Pacific) says groenlandicus' base is smooth and top
> of whorls "glassy smooth." So far, this is not a very comfortable fit.
>
> OK, when I looked in Rehder's National Audubon Society Field Guide to
> N. American Seashells, there was a nice resolution (not necessarily
> correct). Rehder describes M. striatus: "....body whorl with low,
> obscure, distant, wavelike ridges below the suture. Base with lower
> and broader spiral threads...." He also contends that M. groenlandicus
> "however, is smooth and found only in northern Greenland and some of
> the Arctic islands."
>
> I have a feeling this has been a very thorny tangle in the literature.
> Is there any better consensus lately?
>
> David Kirsh
> Durham, NC