The mollusks generally produce a multi- layered shell where in the brachiopods one rarely observes more than one layer. The aragonitic portions are usually removed by dissolution to form a moldic pore or they are replaced by the more stable calcite. The crystal lattice of each rod is slightly rotated relative to the adjacent rods such that when viewed under crossed polarizers one observes a sweeping extinction along the length of the shell as one rotates the stage. The ostracodes are really small and delicate. Look for the sweeping extinction and look for the delicate arch shape of the shell and in well preserved samples one can see a hook or hinge where the two halves of the shell joined.
Trilobites are much larger than the ostracodes.
The sweeping extinction is very pronounced. Barnacles are much more complex in their shape and do not always have a well-developed sweeping extinction. Corals present some challenges. Paleozoic forms secreted calcite whereas the Cenozoic forms produced aragonite. Next the chamber wall is characteristic.