I have some that are quite similar, and the consensus was that they were of the Nassariidae family, some species shown here:
http://seashellsofnsw.org.au/Nassari...idae_plate.htm
Mine have been around a long time, hang out under the rock vs the sand like some of the more common nasarrius. They breed like rabbits and do show up on the glass at night. They have never done any harm, and are definite carrion eaters for one.
