To : Any of you with expertise in fossil shells:

I just returned from a trip to Kino Bay Sonora Mexico.   While making my
rounds of the local shell sources I was given what I assume to be a fossil
murex of some sort.  I have no knowledge of this shell and don't see anything
similar in any of my references.  It is encased in a concretion but where it
is exposed you can see evidence of the shell underneath.  It measures 40mm
end to end, 38mm side to sideand 29mm top to bottom.  I have posted some
rather poor scans of it on PhotoPoint
http://albums.photopoint.com/j/AlbumList?u=515554  the album is Whats this?
 Any help you can give me will be much appreciated. Thanks in advance.

Jim Emmett