Dear Peter,

It's always tough to form an adequate mental image of a species from a
verbal description. As a possible means of narrowing down the possibilities,
take a look at the following page, and see if any of the pictured shells
look similar to the shells of your specimens.  Your exact species may not be
there, but if you can find something similar to yours, then perhaps someone
might be better able to offer a suggestion as to what you might have. The
pictures on this page can be enlarged by clicking on them.

Regards,
Paul Monfils

http://www.seashell-collector.com/beginners/family_id.htm