The Phylum Brachiopoda was immensely important in Paleozoic seas. While a few species live today, most of their former niches have been usurped by the clams.
Like clams, brachiopods are bivalves. Each half of the shell is called a valve. However, brachiopod shells differ from clam shells. Most clam valves are mirror images of each other, like a pair of hands. Most brachiopod valves are not mirror images. While the two brachiopod valves differ in shape, each individual valve is bilaterally symmetrical. If you draw a line down the center of a brachiopod valve, the two halves are mirror images. This is not true for the clams.
In some brachiopods, the two valves fit into each other, sort of like a pie plate on top of a bowl. This kind, such as Peniculauris, rested on the sea floor raised up on its many spines. Others, like Composita, were attached by a fleshy tube called a pedicle.