Hello Anne,
    in reply to your son's question, Knobbed Whelks can grow up to 9 inches
in length.  Channeled Whelks top off around 8 inches.  They represent the
east coast's largest gastropods (snails). They (and their eggs cases) are
also fairly common around Cape Cod where I collect them.  I have from time
to time snorkeled in shallow water and found the large females depositing
the egg cases in the sand (always a remarkable sight).