INTRODUCTION TO THE BRACHIOPODS
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 (see page 16).
Peniculauris (left two), Composita, right
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.