I read some messages about Xenophora being a favorite - and its a
favorite of mine.  They are fascinating and certainly wonderful shell
collectors - actually collectors of a lot of things - coral, stones,
slag, glass, bottle caps, even coins!
 
About your question on why they place bivalves as they do, Dan, it is
much easier to cement a shell with the convex surface to their shell
than the concave.  A friend of mine once had a Xenophora conchyliophora
in his aquarium.  It picked up a 'kitten's paw' which was still alive
and cemented it in place - in about 15 seconds.  The Plicatula gibbosa
lived for about a month, and eventually the top shell fell off.  I
wonder if some of the bivalves they attach aren't alive when cemented
on, too.
 
If anyone knows of a speciment of X. neozelandica kermadecensis being
available, please let me know or refer them to me.  Thanks!
 
Doris Underwood