Hi,
Two possibilities come to mind for your banded, periwinkle-like shells from
Cape Cod.  If they are quite sturdy, about as wide as tall, and spirally
ribbed, they are probably juvenile Littorina saxatilis (Olivi).  If they are
thin, lightweight, somewhat taller than wide, and smooth, then they are
probably Lacuna vincta (Turton).  The Littorina can be found on rocks or on
seaweed.  The lacuna is usually found on seaweed.  In fact, the best way to
collect them alive (and a surprising number of other micromolluscs) is to take
a white plastic pail and half fill it with sea water, then lower a big handful
of seaweed into the bucket and thrash it about violently.  You'll be surprised
how many small shells appear in the bottom of the bucket.  Another method,
slower but requiring less physical energy, is to fill the bucket with fresh
water and lower some seaweed into it.  After a while the little snails will
lose their grasp and fall to the bottom of the bucket.  However, I don't know
about you but most of the places I collect have no ready source of fresh
water.
Regards,
Paul M.
Rhode Island