Guido Poppe has a couple of nice pictures of the [formerly] "missing
shell" on his web site.  It certainly does not look immature, nor
does it look like a Marginella.  To see the images, go to his site
at:
http://www.club.innet.be/~year0078/
click on gallery
click on Cypraeidae
scroll down to subfamily CYPRAEOVULINAE
under which you'll find Genus Chimaeria, and a couiple of great
pictures of Chimaeria incomparabilis
 
Paul M.