Found a pretty nice big shell beachcombing....need ID help.

[bob]

Hello eveyone,
The wife and I were beachcombing in Ventura Ca. last Sunday at low tide and found this shell. Just the very top was sticking out of the sand.. I went to pick it up but had to dig a bit because of it's size...5"
 It was empty so we brought it home....I've been soaking it in bleach and water to clean it.
Can You tell us what it is and what might have lived in it ?
 Also how long would it take to form ? Thank you for your expertise.
<img src="http://i215.photobucket.com/albums/cc138/35Ford/seashell001.jpg" alt="http://i215.photobucket.com/albums/cc138/35Ford/seashell001.jpg" style="border:0" />
<img src="http://i215.photobucket.com/albums/cc138/35Ford/seashell004.jpg" alt="http://i215.photobucket.com/albums/cc138/35Ford/seashell004.jpg" style="border:0" />