my guess from that description would be stomatella varia:

http://reefcentral.com/forums/showthread.php?s=&threadid=162677

Peggy Williams wrote:

>Here's an interesting problem. I'd like to know the answer too!
>
>Peggy Williams
>
>----------
>From: [log in to unmask]
>Date: Mon, 10 Mar 2003 03:32:54 EST
>To: <[log in to unmask]>
>Subject: Marine Snail Identification
>
>Hello, I'm hoping you can help me.
>
>I found the "Conch-L ID Assistance Page" website by searching Google.  I am
>not a Conch-L member, I do not know what it is really.
>
>I have a micro-reef aquarium, and I bought some polyps (coral) from my local
>fish store.  I noticed two very small snails on it when the polyps closed
>for
>the evening once, one with a light gray shell and one with a darker one, and
>did not see them again for a few weeks.  When I did notice them again they
>were almost an inch in length.  Now they are breeding and I have 3 distinct
>generations, determined from their sizes.  I have written to my reef-keepers
>e-group and there has been no help, only one other individual who has seen
>them in his tank, but they are not breeding in his setup.
>
>This is their description:
>    The body (flesh) is almost an inch, a dull gray color, similar to the
>sand in my tank.
>    The shell is helmet shaped, light gray with speckles of either light
>gray
>or a darker gray.  The shell only covers about 1/3 of it's body, with the
>remaining 2/3 uncovered areas totally behind the shell.
>    There appears to be hair-like appendages around the shell on the body
>which at times wrap up around the shell before it moves.
>    The antennae are very long, almost an inch, reaching far out beyond the
>head, very thin and nimble.
>    The eyes are two dark spots that remain close to the shell, they do not
>extend.
>    As the lighter colored one grew, I noticed that there was an indentation
>behind the shell on it's fleshy body, in the shape of an inverted V.  It's
>as
>if when viewed from the side the slope, from back to shell, is interrupted
>just before it reaches the shell, where the body is depressed.  Viewed from
>the top, the depression starts at a point and widens as it disappears
>underneath the shell.
>
>Some of the spots on the shells are perhaps starting to resemble a spiral
>shape.  Without a digital camera, I do not have a way to ask around.  They
>may be a dangerous thing to have around, or they may be a very good resource
>and breeding them can help me or interested marine biology groups.  When I
>found your page, it seemed that your group might be able to help.  I am not
>sure, if I have taken your time inadvertently, I apologize in advance.
>
>Peter Kafalenos
>314-843-9425
>
>PS -- Would you know of any books I could obtain from my library that could
>further assist if you do not know?  Thanks.
>
>
>