I have several "dream shells", but the top three have to be:

1) "Lishkeia" ottoi (Philippi, 1844): Abbott, 1974 lists this as
"common" - right, and so are pots of gold at the end of rainbows.  There
is not a single live specimen in any collection in Eastern Canada - a
"subfossil" is the best i've seen.  David Delucia once got one for $5,
but he seemed reluctant to part with it...

2) Volutomitra groenlandica (Muller, 1842): ok - Abbott has this one as
"uncommon" - to off Massachusetts!

3) Seguenzia monocingulata monocingulata Seguenzia, 1876 - Gulf of Maine
to Mexico - DEEP!!

Surprisingly, i also lack Lacuna pallidula (da Costa, 1778) - it is
supposed to be rather frequent, to Conneticut, in shallow water, on
Fucus - but i just can't find the beastie!!

O yes - i wouldn't mind an Omalogyra planorbis (Dall,1927), - this
little guy is only 1mm or so all grown up - or an Omalogyra atomis
(Phillipi, 1841), which is only half that size in its old age, ad occurs
to Rhode Island, in "lower rock pools".

These are for my study & reference collection, so i don't suppose anyone
will mind if i scream that

I WILL TRADE ***VERY*** GENEROUSLY FOR ANY OF THESE CRYPTIC CRITTERS!!!

-Ross.