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" />

[paul monfils]

This is Astraea undosa, also called Lithopoma undosa, the "waved (or wavy) turban shell".  The animal that lived in it was a "gastropod" or in layman's terms, basically a snail. When alive it has a deeply sculptured "door" or "operculum" that tightly closes the aperture of the shell when the snail withdraws inside.  I don't know how fast this species grows, so I don't know how many years it would take to attain this size, which is about as large as this species gets.  

The bottom image on this page:

<a href="http://www.gastropods.com/2/Shell_292.html" target="_blank"><!-- m --><a class="postlink" href="http://www.gastropods.com/2/Shell_292.html">http://www.gastropods.com/2/Shell_292.html</a><!-- m --></a>

shows a shell with the operculum in place. That shell also has the fibrous outer covering that the animal produces, called the "periostracum".  The top image shows a shell like yours, with the periostracum removed with bleach (though your pictures of your shell do show some remnants of periostracum).  <span class="petit">--Last edited by Paul Monfils on 2008-12-24 19:13:29 --</span>

[bob]

   Thanks, Paul

 Do you know how the shell is formed by the snail ?

  It looks like it was formed in stages with the upper 2 sections being the most colorfull.

 

[paul monfils]

The shell is formed by a specialized organ called the mantle, which extracts minerals, primarily calcium, from sea water, and turns them into mineral salts which it secretes in crystalline form along the edge of the aperture.  This allows the snail to continuously enlarge its shell as it grows.  The growth of the shell is in the form of a widening spiral around a central axis, with the oldest part of the shell at the apex or pointed tip, and the most recently formed shell around the rim of the aperture.  The mantle also secretes pigments that give the shell its colors and pattern.  As you observed, in many species the pigment deposition gradually decreases as the animal gets older (hmmmm, sounds familiar doesn't it?), so that the older parts of the shell are often more colorful than the newer parts.

[bob]

  Interesting,  Thanks again, Paul