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
  http://www.badongo.com/pic/549197
  http://www.badongo.com/pic/549200
2) Mud whelks?
  http://www.badongo.com/pic/549204
3) Rock oysters
  http://www.badongo.com/pic/549205
  http://www.badongo.com/pic/549212
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

The first one is Thais bufo (Lamarck, 1822) (family Muricidae)

The second one is a Clypeomorus (family Cerithiidae). I'm not certain of the exact species. There are a number of these that are quite similar. Clypeomorus petrosum (Wood, 1828) would be my best guess. You may find this listed in various sources as Cerithium petrosum or Clypeomorus petrosus.

The third one is, as you said, an oyster (family Ostreidae). Again not 100% certain of the species but it's close to Saccostrea cuccullata (Born, 1778), which does occur in South Africa.

Thank you for the quick response. The information certainly helps. What an amazing website!