It is a moon snail egg case (here in the Northwest from Pollinices).
The eggs are not laid in the center like a bird nests, but are mixed in
with the sand and mucus.
 
> ----------
> From:         Leslie Shattuck[SMTP:[log in to unmask]]
> Reply To:     Conchologists of America List
> Sent:         Wednesday, June 10, 1998 1:16 PM
> To:   [log in to unmask]
> Subject:      Sharks nest
>
> Hi shellers,  I am looking for the "parent" of what we used to call a
> "sharks
> nest" found around Newport Beach bays, in calm ,shallow water.  It is
> a collar
> of sand cemented togeather with some sort of mucus, with a hole in the
> middle.
> As kids, we used to think sand sharks made them for their eggs.  But I
> have
> heard that the moon snail really makes them for their eggs. Does
> anyone know
> about this curios little nest? Thanks ,Leslie
>