Another Strange Sea Creature

Mr Flum, aka Little-Shirley-Sharp-Eyes, noticed this cone-shaped shell on the beach. Most shells we can place in one family or another but this is a strange one. The full length is about 8inches, the coin, a 50cent NZ, equates to a 10p. We found others broken up, so they are not uncommon, just new to us.
Also found another sea-slug but left it well alone: my shoes and socks are barely dry from yesterday's dipping!