Lampshells (brachiopods) may resemble small clams or mussels, but these animals are actually very different than bivalves.  One difference is that lampshells are attached to the substrate by a stalk or peduncle.  The other main difference is that brachiopods have lower and upper shells, unlike bivalves which have right and left-sided shells. The anatomy of lampshells is also very different as they have a large filter feeding organ (lophophore), but do not have gills like clams or mussels.  There are fossil records of lampshells living in the Cretaceous oceans millions of years ago when they were very successful, but only a few species exist today.