Requesting id Madagascar south coast marine shells

[ebryant]

I have three species of shell which we are trying to radiocarbon date to get the age of a large tsunami event about 4000 years ago in southern Madagascar. Unfortunately the accuracy of the date depends on knowing the species of shell. Photos of the species are as follows:
1) Molluscs
  <a href="http://www.badongo.com/pic/549197" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/549197">http://www.badongo.com/pic/549197</a><!-- m --></a>
  <a href="http://www.badongo.com/pic/549200" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/549200">http://www.badongo.com/pic/549200</a><!-- m --></a>
2) Mud whelks?
  <a href="http://www.badongo.com/pic/549204" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/549204">http://www.badongo.com/pic/549204</a><!-- m --></a>
3) Rock oysters
  <a href="http://www.badongo.com/pic/549205" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/549205">http://www.badongo.com/pic/549205</a><!-- m --></a>
  <a href="http://www.badongo.com/pic/549212" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/549212">http://www.badongo.com/pic/549212</a><!-- m --></a>
All shells are about 3.5-4.0 cm in length, about the size of the "Enter" key on a computer keyboard
Any information would be greatly appreciated.
Thanks
Ted Bryant