Call me deranged or call me subversive, just don't call me late for . . .
oops, I digress . . . anyway, neither those prickly things not those
fan-shaped things have ever ranked among my favorite bivalve groups.  I'm
kind of partial to Veneridae (Venus Clams), with Cardiidae (Cockles) a close
second.  I also find Carditidae interesting.
The only large shelled molluscs I have ever heard of from the deepest parts
of the oceans are bivalves.  Some of those thermal vent clams found along
the mid-oceanic ridge are a foot long and a couple of pounds apiece.

Paul M.