Joan
 
Barbatia, like the Arca are extremely variable in shape but the hinge
characters, rib sculpture and periostracal hairs and layers are very
good methods for distinguishing the species.
 
If you want to get serious about the group, let me know and I will send
you a list of reprints available from your local Museum or Uni, dealing
with the family, we certainly need more researchers in the bivalvia.
 
Kev Lamprell