Useful Rules for Identification
Brachiopods and Bivalves

When working with fossils having two shells, remember one of the useful
"rules" of fossil identification: that brachiopods have a symmetry between
right and left sides of a valve, whereas bivalves have a symmetry between
the valves. With brachiopods, the ornamentation is a mirror image from one
side of the shell to the other side. Also, the beak is centrally located
and points towards the hingeline, not towards one end of the shell. With
bivalves, the beak if often located off-center and usually points towards
one end of the shell. This "rule" works in many situations, but is not
always valid, so apply with caution.

Another caution to remember is that often the shell is incomplete, with
some part of the shell missing or obscured and not visible. This may be a
bit confusing when trying to make an identification, but incomplete
specimens are the norm among fossils; complete ones are the exception.
Therefore we need to have a mental image of the complete specimen when
doing identifications. And to avoid confusion, one should also have a
mental image of what the specimen would look like if it is worn by
weathering or solution etching. However, on specimens with areas of good
surface, minor or trivial details of ornamentation or shape may allow a
worker to recognize a fossil.

Dr. Tom Yancey

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St. Louis Pennsylvanian Fossils of the Altamont Formation